Hello, I'm Gwen Ladybug, a recent low elo Gwen OTP. I'm making this guide mostly for my eyes only, but since Mobafire doesn't let you make your guide private, might as well make it anyway for others.

Strenghts

- Can play at 3 different roles (top/jungle/mid)
- Rarely picked or banned (at least in low elo)
- One of the best AP scaling champion (if not the best except for Veigar)
- Excellent teamfight power
- Annihilates tanks
- Super easy to farm with
- Very good at pushing and destroying towers/objectives
- Good sustain paired with one of the strongest defensive spell of the entire game
- Blue and cute

Weaknesses

- Terrible early game
- Not very good at engaging teamfights
- Vulnerable against high mobility champions
- Correctly using W can be very hard

Passive: A Thousand Cuts

"Gwen's basic attacks on-hit, the center of Snip snip, and Needlework deal bonus magic damage equal to 1% (+0.6% per 100 AP) of the target's maximum health.

Heals Gwen for 50% of post-mitigation damage against champions, capped at 10-26.76 (+6.5% AP) per instance."

Gwen's absolute bread and butter. This passive is what makes Gwen an anti-tank champion, a monster of sustain, and just a good champion overall, and is the reason why stacking AP on Gwen is so good. The more AP you have, the more damage to max health you deal, and the more you are allowed to heal with each instance, it's that simple. It also deals bonus damage to minions under 40% HP, which helps with last hitting.

Q: Snip snip!

"Passive: Gwen's basic attacks generate a stack of Snippy on-hit which lasts 6 seconds, stacking up to 4 times and refreshing on subsequent attacks.

Active: Gwen snips at least twice with her scissors in a cone in the target direction over the cast time, dealing magic damage per snip to all enemies in the area, with the final snip dealing increased damage. The center of each snip converts 50% of the damage to true damage, then applies A Thousand Cuts. Deals 80% against minions and executes them on the first snip if they are below 20% health."

Snip snip is your main damaging ability and your main trading tool. The true damages it deals are no jokes, and it applying the passive is incredibly strong as the game goes on. Be careful not to be too deadset on using it only when you have 4 stacks; it's better to dish out some decent damage than taking a bad trade or dying because you held it. Its little range goes farther than your AA range, so don't hesitate to use to last hit some minions while remaining safe. Also be careful as it can push the wave extremely hard, so be mindful of that when using it to trade.

W: Hallowed Mist

"Gwen summons the Hallowed Mist upon her current location, lasting for 4 seconds. Hallowed Mist can be recast after 0.5 seconds, and does so automatically if Gwen attempts to leave the area.
While inside the mist, Gwen becomes ghosted, gains bonus armor and bonus magic resistance and is untargetable to all enemies outside of the mist.
Recast: Gwen commands the mist to move to her current location."

By far Gwen's most infamous spell, but also the hardest to master. Hallowed Mist basically allows you to have a duel with one ennemy during a teamfight. You can see it as some kind of little Xin Zhao ult in a way. I'd like to call your attention to the fact it gives bonus resistances, as it's a very unknown part of that spell. Indeed, the mist is not just good against ranged champion, but also allows you to temporarily be extremely tanky when facing a melee champion, even though an excellent Gwen player will be able to position in a way that makes him untargetable even to melee champs.

E: Skip'n Slash

"Gwen dashes to the target location, then empowers her basic attacks within the next 4 seconds to deal 15 (+20% AP) bonus magic damage on-hit and gain bonus attack speed and 75 attack range.
Gwen's first basic attack within the duration refunds a percentage of Skip'n Slash's cooldown."

Gwen's mobility and farming tool. This spell is incredibly good and allows you to hit Snip Snip! on ennemies "by surprise". You can E -> Q, but also know that if you Q first, you can dash anytime while you're snipping. This can be useful to let your opponent think you're just farming or to reposition to hit more snips. In early, the CD is rather long and the refund from the first AA after it is quite low, but in late game you can basically dash all the time. This helps Gwen with sticking to her target and the attack speed steroïd is just massive in late game. Basically, this spell is a Nashor's Tooth on a stick, as it has roughly the same on-hit damage and AP scaling. Also note that you can go through walls with it, but it can be tricky with some walls. This is an AA reset.

R: Needlework

"Gwen launches a needle in the target direction that deals magic damage to enemies hit and slows them for 1.5 seconds.

Needlework can be recast twice more within 8 seconds of the activation at no additional cost, with a 1 second static cooldown between casts. Each subsequent cast refreshes the duration and increases the number of needles by 2. Needle hits before the first apply a slow against each individual target."

Gwen's only CC (a cute little slow), and main reason she's so dangerous in teamfights. Each needle procs A Thousand Cuts, hence each target you hit per needle is an instance of healing! This spell is simple yet extremely versatile. You can use it to chase targets, survive by slowing enemies down/healing, or just finish someone. Don't be tricked into using ultimate related gears like Axiom Arcanist or Malevolence; They won't do much (Malevolence is a terrible item) since your ultimate's damages are not one big instances but many little ticks of damage, and most of its damages come from your passive! So these won't increase it's damage whatsoever.

Keystones

Conqueror is without a doubt the best possible keystone to use on Gwen. The stacking AP it provides rewards long fights, which is exactly what Gwen is designed for, and the 8% omnivamp it offers once fully stacked is just extremely good combined with Thousand Cuts healing (which itself is amplified by the AP given by Conq). Even though it's obviously weaker in early game, most of the time you can duel your opponent if they use Hail of Blades. Each time you choose another keystone, you have to weigh the pros and cons of not running Conq. Another good thing is that Gwen uses the Precision tree really well.

PTA offers you a quick burst compared to Conqueror that punish your opponent if they try to continue fighting thanks to the 8% damage increase you get after procing the 3 marks. It's better than Conqueror in early game, but it tends to fall off while Conq will scale very hard, and the 8% damage increase doesn't apply to true damages. Not to mention you pass on 8% omnivamp. You can try it if you want to have an easier early game, but it's way worse than Conqueror to me.

If you wish to play Gwen on-hit, run it, but just like PTA, it will fall off hard in late game. Gwen being an AP only champion, she doesn't have access to a lot of attack speed/on-hit items compared to AD or hybrid champions like Kog'Maw. Try it for fun but it's not that good.

Fleet is made to sustain in lane and have a hard time against your match-up. However, Gwen already sustains from fighting champions and has a rather easy time surviving the lane thanks to W and E, I'd advise not to use it at all.

Domination

Electrocute and Dark Harvest can both be used, but they're better suited to Gwen Mid, as they won't be that effective in most toplane match-ups. Note that Electrocute might be a bit tricky to proc compared to Dark Harvest though.

Hail of Blades might sound like a good idea: after all, you dash in, do 3 AA that also deals some true damage, and Snip Snip!. Actually, it's pretty terrible. While Conqueror value keeps increasing as the fight go on, HoB's is non-existent after the 3 (4 if you keep your E to do an AA reset) first AA. Try it at your own risks.

Sorcery

The new version of good old Phase Rush. It's a bit harder to proc (at least for Gwen in early) but it's compensated by the fact the cooldown is much lower as the game goes on. If you manage to use it correctly, you could use it against Darius to engage, trade him quickly, and go back while negating his slow. I couldn't actually make it work so far, but it's an interesting idea.

Resolve

Against a very heavy tank like Mao'kai, you could argue taking Grasp to match his HP scaling. But since Gwen is so good at shredding tanks, I think Conqueror is better since it will amplify Thousand Cuts damage to their max HP. However, if your team has no frontline, it might be possible to go for a more tanky build on Gwen and sacrifice some damage to be a damage dealing frontline.

Inspiration

The go-to rune for Gwen mid and Gwungle assassin build. First Strike is very much overrated actually. The 7% damage increase might sound nice, and it is, but in case you didn't know, it used to be 7% bonus TRUE DAMAGES on all of your damages. The gold generation it gives is also pretty bad. It's only real good side is that it's easier to proc than Electrocute or Dark Harvest, as Gwen can easily walk up in her mist and dash before taking damage.

Minor runes

Precision

The Precision tree is excellent for Gwen. It's designed around long sustained fight, and that, as a magic blue doll, is exactly what we want, that's why we're playing in toplane.

You can run any of these 3 to your convenience. Absorb Life should be better if you're struggling in lane against that match-up, but I personally don't like this rune. The sustain it offers is awful in early game, and while Gwen can farm quite safely thanks to her kit, I think you're better off using the other 2. Triumph can be handy if you plan on long, hard team fights/skirmish, since the regen it will offer you can help you continue fighting. Presence of Mind is the default rune you want tho. It allows you to use more spell, and that's exactly what an AP champion like Gwen wants, especially since Gwen has very short cooldowns pretty quickly. If you also run Doran's Ring, you're guaranteed to never run out of mana.

Again, choose whichever you prefer. Alacrity is way better than Haste. Attack speed is a great stat on Gwen but you don't want to build Berserker's Greaves, and it adds more value to the minions you're farming since they bring you closer to the completion of the rune. Haste can be nice, but Alacrity is definitely better suited for Gwen in my opinion.

Each of these 3 runes are great! Coup de Grace doubles down on Shadowflame passive and leaves less room for your opponent to escape with 1 HP left, it might be a good option if you're up against a squishy champion. Last Stand works very well with Gwen too, as it does with all champions that sustain a lot. It can be a nice option if you know you're going to have a hard time in lane and never be full HP. Cut Down is definitely my favourite. It makes you threatening as soon as level 1, and helps you reach the threshold for Shadowflame passive to operate. Basically it gives you an edge at the start of the fight that compensate the time Conqueror takes to be stacked.

Domination

Personally, I don't think Gwen has much use for the Domination tree. The "vision" runes are pretty bad on Gwen, even though you can argue that Deep Ward is nice on pretty much any champion, and the Hunter runes are pretty bad when you're playing toplane, since you won't stack them before a while.

Taste of Blood offers you some sustain from time to time if you're able to damage your opponent. The thing is: Gwen already sustains by fighting enemy champions anyway, so why take a rune that does just that but with a cooldown? Sudden Impact on the other hand can be actually good. It adds a little extra true damage punch to your E, so if you REALLY want to use the Domination tree, use this one.

Sorcery

The Sorcery tree is actually pretty interesting for Gwen. It's the one I'd go to if Resolve wasn't that good on her.

The 3 are valid options on Gwen, but there are some nuances I'd like to tell you about. Axiom Arcanist works, but not the way you'd think. Actually, the damage increase it offers is terrible on Gwen. First, Needlework is an area of effect, which means the damage increase is only 8%. Secondly, even though the needles do some nice damage by themselves, keep in mind that it's actually many instances of small damages put together, so a % increase in damage won't do much. Lastly, The real damage of your ultimate actually comes from Thousand Cuts, which won't be increased. If you run it, it's because you're more interested in its second effect: refunding a part of your ult cooldown on kill/assist. It's really not that good, but if you like it, use it. Manaflow Band is a very good rune. But not really on Gwen. You can try using it if you don't have Presence of Mind and or Doran's Ring, but it's definitely more of a "comfort" rune. Nimbus Cloak is the most interesting of the bunch. Gaining move speed when popping a summoner spell is incredibly good: your Flash combines with a mini version of Ghost and so on.

Transcendence is a nice little scaling rune that will bring you a lot of haste as the game goes on. One might argue that it gives less haste than Legend: Haste, but you don't have to stack it as it will scale with your level, and at level 11 you get the added bonus of refunding part of your CD on takedowns, which is better than what Legend: Haste offers. Absolute Focus can be used, it's basically a reverse version of Last Stand, so the 2 go well together. However, this one gives you AP rather than increasing your damages directly. I've never been fond of this rune on anyone but you can use it if you like that playstyle.

These ones are quite simple. Scorch is better fitted if you plan to dominate the early game and bully your opponent, but it will fall off hard in late game. Waterwalking helps you move faster around the map and gives you some AP when in the river. If you want to help your jungler and be more dangerous around neutral objectives or if you're the type of toplaner to roam mid, definitely give it a try. Gathering Storm helps you scaling. Games in low elo tend to be 30+ minutes, so it will definitely give you a lot of AP for free! The best of the 3 in my opinion.

Resolve

The absolute best secondary tree in my humble opinion. Gwen often struggles in early game in toplane, so having a secondary tree that helps her survive the lane is always nice.

Even though Gwen is not a crazy HP stacker, she's excellent at snipping towers down. Having a rune that further helps her in doing so turns her into an absolute wrecking ball. I often swap it for something else, but in free match-up you can take it and try splitpushing if your team struggles in teamfight.

The "Match-up trio" as I call them. Conditioning is a good rune if you want to scale your resistances. It further increases the stats your mist give you and adds some value to your defensive items if you build any. Second wind allows you to bite the bullet more easily against ranged/poking match-ups like Teemo or Vayne. If you take it, be sure to also start with Doran's Shield to make sure poke does nothing to you. You can also use it against Mordekaiser and Darius to reduce the impact of their passives. Bone Plating is the other side of Second Wind: it reduces (heavily) bursts. if you're against a champion that likes to go in, or a Hail of Blades user, this one will most probably win you early trades by itself.

When I plan to scale I personally love using Overgrowth. Having some extra HP is always nice, and if you go Riftmaker, it's a small added bonus. Revitalize can be pretty good too, since part of your sustain comes from Thousand Cuts and it's an actual heal. Be careful, the heal bonus it gives DO NOT APPLY TO OMNIVAMP! Unflinching is my go-to against champions that like to engage with a hard CC like Darius.

Inspiration

The Inspiration tree is full of weird and interesting minor runes. If used correctly they can be very powerful on Gwen.

These 2 runes are very similar in the way they both are economy oriented. Magical Footwear makes you save on 300 golds by giving you free boots at 12 minutes. However, if you manage to kill your enemy, it gets more and more reduced. It can potentially proc earlier than Cash back, but once it has proc, it's done. Cash back on the other hand will generate more and more golds as you complete items. I prefer it over the boots because most times, you can rush your first item and get refunded enough gold to get the boots anyway.

Yes, I never use the biscuits. It's a terrible rune. Triple Tonic is pretty meh too. The only tonic you're interested in is the adaptive force potion you get at level 6. 6 is a big powerspike on Gwen, with the tonic you get a chance to just stat check and kill your match-up pretty easily. Time Warp Tonic is a very good rune. It increases the value of potions, and since you should get a Refillable Potion each game, if you run Inspiration I highly recommend you use it.

Cosmic Insight is pretty much a comfort rune, there's nothing wrong in running it but there isn't much thing to say about it. Approach Velocity can help you chase people hit by Needlework or collapse on people being CC'ed by your allies. If you run Inspiration and face a lot of range or mobile champions, consider using it as it will make your life easier.

Gwen's needs one stat: AP
She also likes having some attack speed, a bit of haste, and HP since in top lane she's mostly played like a bruiser.
First, let's see her 3 options when it comes to first item:

Riftmaker

Although most Gwen mains, and in general most players consider Riftmaker to be a trash item, it's actually a pretty good one, the only problem being it's a misunderstood item. Only a few select champions actually benefit from Riftmaker to its full potential, mainly Mordekaiser and Vladimir. Omnivamp is a rare stat, but like lifesteal, it's only really useful if you have some resistances in your build. In all honesty, Gwen is far from being the best champion to run it, but it's still one of the few items that really suit her. Depending on the build, you're going to have both HP to gain a bit of AP and resistances, not to mention your W already gives you a ton of resistances, even though it's only for a short time. Riftmaker's main weakness is that it takes 4 seconds in combat to gain access to its omnivamp. Even though it's not the flashiest or most powerful item, it's in my opinion the best choice if you're having a difficult time in lane or if facing a hard match-up: it gives you survivability, some damage, and also has a cheap and pretty good build path. Haunting Guise is one of the best components in the game, and synergizes well with Conqueror and Gwen's love for extended fights.

Dusk & Dawn

The new Sheen item, and considered by far the best item Gwen could dream of. It gives extremely solid stats, and a bit of everything. Nice AP, decent attack speed, a lot of health and some haste to top it off. The Sheen proc scales with AP, not as hard as Lich Bane but still can add about 60 damage to your next AA. The double on-hit proc also doubles Thousand Cuts, Nashor's Tooth and your E's bonus on-hit damage. However this item comes with a huge flaw: its god awful building path. The only good component required for it is the Blasting Wand. Classic Sheen is meh on Gwen since it only does damage based on your Base AD, even though it does add a bit of damage anyway. Kindlegem is a TERRIBLE component, and a plain Dagger is also rather awful. I usually avoid building it for now, but I don't think you should too, if you prefer using it over Riftmaker and don't mind its terrible build path, then you should definitely go for it. It allows for faster Q stacking and offers a bit extra sustain instantly compared to Riftmaker. Like the latter, it's debatable whether or not it's actually an interesting first item choice. The pretty low AP ratio of its Sheen proc actually encourages you to build it earlier on (as opposed to Rabadon's Deathcap for example). I'd say it's a nice alternative to Riftmaker if you don't go with the omnivamp boots. Just don't expect the double on-hit to do much in early damage wise.

Nashor's Tooth

By far Gwen's most aggressive item to rush. It gives way more AP than the two others, gives an absurd amount of attack speed, some haste, and a very good scaling on-hit damage. It has a ridiculously strong synergy with Gwen: its high attack speed helps you proc Thousand Cuts more often, it does AP scaling on-hit just like your E that also gives you a huge attack speed bonus. It helps you melt tanks and the on-hit helps you melt squishies in late game. It also has a nice building path with a Blasting Wand and a Fiendish Codex, which allows you to stack a healthy amount of AP during your first backs, and Recurve Bow is way nicer than a plain Dagger. Its main drawback being that unlike Dusk & Dawn or Riftmaker, it doesn't give you flat HP, but rather more instances of passive to heal from, so if you're struggling against your match-up or against a very high burst opponent, it might be a riskier choice.

Boots

As many AP bruisers, Gwen has the luxury of being fine with almost any boots. I consider them to be mostly match-up reliant (match-up as in the whole enemy team):

Gluttonous Greaves

The new boots of omnivamp that are very popular on Gwen (and a lot of other champions). They feel very fun and nice to play with, but the fact you need to stack them with 10 kills/assists makes them more of an easy game oriented choice. With Conqueror and Riftmaker passive stacked, you reach a whooping 28% omnivamp, which is A LOT considering Gwen also sustains off of all her passive's procs and can deal some true damage. The only problem with these is that omnivamp becomes really strong only if you have some HP, which Gwen usually builds, and some resistances. Even though as said above, your mist gives you a ton of resistances, keep in mind it's only for 4 seconds which is not a lot. Another way to see it is when you can't decide between Plated Steelcaps or Mercury's Treads. Anyway they're almost never a bad choice and are really fun to play with if you want to go for a full sustain gameplay.

Plated Steelcaps

The iconic armour boots, also known as the Tabis if you played for a while. They're very straightforward: they give armour and reduce damages you take from AA by 10%. If you're against a heavy auto-attacker team, an AD top/jungle duo, or even just against an auto-attacker in toplane like Tryndamere, or just don't feel confident enough to use Gluttonous Greaves, they're your go-to. The little armour they give is a nice help to your already big sustain, and the passive just makes your life a lot easier against AA oriented champions.

Mercury's Treads

Just like their armour cousin, they're the defensive option against AP champions, and most particularly crowd controls. Honestly, these are far less used than the two boots above for multiple reasons. First, top lane is usually played by AD oriented champions, and secondly, tenacity doesn't reduce displacing effects or bumps. Anyway, don't hesitate to build them if the enemy team is heavily AP or if CCs are a pain for you.

Those are the big 3, you're most likely always going to use them.

Sorcerer's Shoes

Personally, I never use these in top lane. The flat magic penetration they offer is usually not very interesting in top lane, most toplaners are usually too beefy for it to actually matter. You're more likely to use them when playing Gwen mid, as it's a more assassin playstyle, where you're aiming to make squishies explode. You could consider using them when you're against very squishy champions like Vayne.

Ionian Boots of Lucidity

Just like Sorcerer's Shoes I never use them. In late game Gwen already has very short cooldowns on her Q and E. You could technically consider them if you like running Legend: Haste on Gwen (that's possible), mostly to have your ult and W up more often to be safer, but I think that's more interesting to just use defensive boots instead. I advise you to ignore them. One argument in favour of these tho: they give summoner spells haste, which can come in handy if you're relying on these for your laning phase/game plan, but even that is a bit meh to be honest.

Boots of Swiftness

The fastest of boots, they feel nice to play with and can be useful if the enemy team has a lot of slows, or a lot of ranged champions who don't depend on AA (which is very unlikely). For the latter, Ghost will probably be a good enough solution.

Berserker's Greaves

If you want to play Gwen on a more attack speed on-hit build, you will most probably need them, along with Lethal Tempo, but be wary that on-hit Gwen usually feels very underwhelming, see it more as a fun build to try something new.

Other core items

Shadowflame

Once you have your first item and your boots, you should always go for Shadowflame. This item is probably in the top 3 best AP items in the whole game: a ton of AP, flat magic pen, and a passive that's incredibly strong, Gwen being one of the rare AP champions to benefit from the bonus its passive gives to true damage. It helps you finish of targets, and makes Cut Down way more threatening as it will help you get to the 40% health threshold required for Shadowflame's passive to proc. If you prefer using Coup de Grace, then it will double down on it. Just an all around great item for duels and becoming dangerous towards squishies. Note that if you started with Riftmaker, you might want a Nashor's Tooth or Dusk & Dawn first to have some attack speed.

Rabadon's Deathcap

The absolute turning point of your game: once you have it, everyone melts in front of you basically. Thousand Cuts starts dealing an absurd amount of damage, letting you destroy tanks and bruisers, and your damages just make squishies go boom. The item is very expansive, but it's worth it. BUT please, always build it once you have at least 2 other items (boots don't count). Rabadon's passive increases your AP by 30%. That's massive, therefore if you build it too early, your golds will be spent on a super expansive item that doesn't offer that much at that point in the game. Shadowflame gives almost as much AP, but you also get flat magic pen and its powerful passive.

Situational and defensive items

Void Staff, Cryptbloom and Bloodletter's Curse

The 3 %magic penetration items available to you. Gwen can use any of these. Void Staff is obviously the most agressive one, it gives more AP and 40% magic pen, while the other 2 only gives (or reduce magic resistance by) 30%. Cryptbloom and Bloodletter are more team play oriented in different ways, but both offers you some HP to help being beefier and increase your AP in case you're running Riftmaker. Cryptbloom will help your team heal in the middle of a teamfight, but Bloodletter can be better if you're playing with other AP champions. The latter will reduce the magic resistance of enemies and help your other AP allies deal more damage to them. That reduces the value of their own %magic pen, but it will still do more damage all in all.

Zhonya's Hourglass and Banshee's Veil

When it comes to items that gives both AP and resistances, those are the only 2 available sadly. I'm not a fan of any of them. Zhonya's 100 AP might look impressive, but don't forget it only gives 50 armour with no HP at all. It's an item you want mostly for it's active, and unlike Swain or Fiddlesticks, Gwen doesn't have an ability that will do damage while she's in stasis. Banshee is even worse; it gives only 40 magic resist, which is especially bad since almost every champions gain only a few magic resistance per level compared to armour. The spellshield Banshee offers is also quite bad against most teams and might feel overkill on Gwen, since the mist will be more than enough to avoid annoying spells. I personally almost never use Zhonya and never build Banshee. I'd recommend going for more traditionnal defensive/tank item if you're looking to survive.

Morellonomicon

While anti-heal is maybe one of the strongest effect in the game, only Mortal Reminder is actually worth it as a legendary item. If you're looking to stop your opponents from healing, just build an Oblivion Orb and let it sit in your inventory. Morello is just not worth it.

Lich Bane

Lich bane is a very fun item to use, but keep in mind that if you build it, you have to sacrifice building Dusk & Dawn. The problem with it is that in toplane it might be too "bursty" to have an actual impact, as it is mostly designed to be good against fragile champions. You'd want to build it more on Gwen mid.

Liandry's Torment and Blackfire Torch

I haven't tested any of these burn items on Gwen, but both have some interest. Liandry is a very good item, and it has the same passive as Riftmaker, which combined increase your damages by 14%! It could be an interesting pick against very HP heavy teams, but I'm not sure Gwen would be the best champion to use it, as she already has everything she needs to shred tanks/bruisers. Blackfire Torch's burn on the other hand scales off of your AP AND gives you 4% AP per enemy burning because of it, which can be pretty interesting in late game. Proc the Blackfire burn on the whole enemy team with Needlework, and enjoy the +20% AP!

Jak'Sho, The Protean

Jak'Sho is by far the best defensive item you can go for in late game when you don't know which to pick or when the enemy team is 50/50 in types of damage. It gives everything you need, HP, armour and MR. Once the passive procs and increases your resistances by 30%, your mist increases the value of the item greatly. Be careful tho, as you might not be tanky enough at the start of the fight, but luckily Gwen sustains a lot to make up for it at least a bit.

Force of Nature, Kaenic Rookern and Abyssal Mask

3 solid options when it comes to MR. Force of Nature is the most fun to use, but the problem is that Gwen doesn't have any shield to stack it safely by taking damages over time before the fight starts, even though her sustain can make up for it. The move speed it gives can be nice to help keep up with mobile champions too.
Rookern is by far the most powerful MR item. It gives a lot of HP, an absurd amount of MR (even tho Force of Nature gives technically more once stacked) and a massive magic shield. Even tho the shield scales with max HP, it's not a problem since Gwen builds HP anyway.
Abyssal Mask is an affordable option, more humble when it comes to MR, but it has the nice passive of reducing by 30% enemy's MR when they're close to you. You can see it as an "offensive defensive item".

Dead Man's Plate, Randuin's Omen, Frozen Heart

Deadman's Plate might be a good option if you struggle against slows and for catching ranged champions, I haven't tested/needed it so far but know it's there.
Randuin can be a life saver if you're against a lot of crit oriented champions or if the enemy ADC becomes too much of a problem. It's active also helps you stopping enemies from kiting you too much.
Frozen Heart has the same problem as Zhonya, i.e it only gives armour without any HP. In return, it's the single item that gives the most armour. The passive helps against attack speed oriented champions, but it will probably be less frequent than going Randuin when you need armour.

Cosmic Drive

In super late game, you will probably want to sell your boots for a Cosmic Drive to gain a bit more of this juicy AP. It gives good haste, some HP and the passive gives you move speed to reposition and chase enemies.

Ignite

Ignite is very popular in toplane. It gives you more kill pressure, allowing you to get a kill early even without any item. The 40% anti-heal it applies also help you against champions who sustain hard like Dr.Mundo, Aatrox or Irelia. You can consider it one of your default choice when you don't know what summoner to run.

Ghost

As Gwen, you're one of these champions who don't need a Flash at all. You're looking for extended fights/trades, and Ghost can help you sticking to targets more easily. If you have a good map awareness and know the enemy jungler is coming for you, it's just as good as Flash to escape and go back to safety, with the added bonus that it has a shorter cooldown. Along with Ignite, it's your by default choice for a defensive/utility oriented summoner. Note that against champions like Darius who run it, Ghost is your best bet to escape, while Flash may struggle depending on where you are on the lane.

Flash

By far the most used summoner spell in the entire game, and for good reasons. Flash is so strong it's actually its own counter. I'm not fond of using it when playing Gwen, but it allows some surprise Flash + Snip Snip! to finish your enemy off.

Teleport

Now that the toplane quest gives you a free TP once you reach level 10, you might wonder why not skip it as a summoner and use other sums instead. The reason is simple: as Gwen you will struggle against a lot of match-ups in early. Having the luxury to base and come back instantly with TP is very valuable. It offers you comfort and dissuades you to int by staying in lane with 10 HP. The real TP also has a much lower cooldown compared to its free version given by the quest. Once your quest is completed, TP becomes Unleashed Teleport and offers you a massive shield that is very useful if you TP botlane or somewhere else to help with a complicated teamfight, or to abuse your tempo against an enemy who stayed in lane for too long. I like using it instead of Ignite when I'm not sure about the match-up's difficulty.

Exhaust

I have never used it so far, but it remains a solid option against all-in champions. I have seen people use it against Darius to reduce his trade potential, but keep in mind that the real problem is his ultimate, which deals true damages, so Exhaust won't reduce them at all.

At the start of the game, you're pretty much powerless. Gwen scales very hard, but compensate that huge strength by having a quite bad early. Knowing your match-up is something that will allow you to know whether or not an early fight is possible or not.
Your main goal will be to farm as much minions as possible and avoid dying at all cost. Dying throws games and give your opponent a huge advantage to farm and/or take plates of your turrets. Try to last hit as much as you can, even use some spells if you have to. In case you can't fight your opponent, try to get the wave to freeze right in front of your tower. This way, you can farm safely near your turret and the enemy will have to commit to your side of the lane to get some XP and gold, becoming vulnerable to a gank from your jungler. Be careful that if the enemy manages to shove the wave under your turret anyway, your wave will probably push to his side of the lane, and they will be able to freeze you instead.