Don't force anything with Gnar He'll kite you around with his insane MS and has his hop to get away if you somehow get to him. When he's about to transform, (bar will be almost filled) back away. If he does catch you by surprise, use your Snip Snip! to dodge his stun. Don't kite towards walls easier if he is about to transform. Stay in the middle of the lane and make it hard for him. If you don't force anything, Gnar can't do much besides poke and you'll outscale him as Gwen You will need to be patient in lane which is extremely important for climbing.

Technically, Dark Seal and Mejai's Soulstealer scales with any AP champion. I really recommend just building normally instead of building those snowball items. If you build those items, it's forcing you to play perfectly and you aren't going to be able to limit test as much (which I think is extremely important for climbing). Having 25 Stacks can put a bit of pressure on you which can affect your decision making too. However, if you're really dominating, it is a good item to build because it will give you that AP to end the game faster. You'll be able to force more plays with your strength, but if you make the wrong decision, it will cost you some of your power. If you don't know if you can really play perfectly, just build Dark Seal and see if you can get 10 stacks and keep it for a while. If you can, you should definitely upgrade it into a Mejai's Soulstealer to get more stacks/AP. Remember that Mejai's Soulstealer is a double-edged sword, high risk, high reward!

Doran's Blade should always be your go to starting item. The other two are more situational in my opinion. Corrupting Potion should be taken when you are using the Time Warp Tonic rune in the inspiration tree. Basically, the other rune page on this guide. Doran's Shield should be bought into champions like Tryndamere, Nocturne, etc. Doran's Shield allows you to tank more of their burst trades and stay in lane longer because of the regen passive you get from it.

I really don't recommend taking runes like Time Warp Tonic in the inspiration tree because runes like Revitalize in the resolve tree offer so much more and actually scale into late game. Most of the runes used in the inspiration tree are more orientated on early game so if you think you can snowball from laning phase, go for it! You'll have an easier time with those runes. Otherwise, I think Gwen can hold her ground without the inspiration tree because her early game is strong enough.

I may not be Grandmaster, but I have over 200k Mastery points on Gwen (which is one of the highest in the world right now), so I think I can say, I'm one of the best at her mechanically wise. You don't have to take my opinion if you don't want to, but this is how I view these starting items on Gwen.

Sorry for a long response but...

I've gone against TONS of Mordekaiser's as Gwen and once you know how to beat him, its easy. Set up the wave and freeze it under/near your tower. Remember this, he wins extended trades because of his passive and his shield he will get. If Mordekaiser tries to walk up and Q the wave, get ready to immediately E onto him. Abuse the extra 100 range you get and kite him around. End the trade with your Q and back off. Never engage Mordekaiser first, just play patiently and scale for free. Once you get one of your Mythic components and Ultimate, your free to all in-him. You should try to time a back when your just about to hit level 6 and when you can get one of your mythic components. Once you get back to lane, you should be level 6 before him if you did a good job zoning him off. All-in him as soon as you get 6 by engaging with your first Ultiamte charge and E. From here just finish him off and volia, you got a lead on him. From here, you can just continue freezing under/near your tower and killing Mordekaiser when your Ultimate comes up if he tries to walk up for CS. If he gets a grab on you and he's looking to follow up with his Q, dash towards him. This will pull you through him and put you in the perfect position to take a decent amount of health from him. When you fight Mordekaiser mid-game you don't need to be that afraid of him. Your E should be up everytime his Q is up so you can simply dodge his ability and negate half of his DPS. If you dodge his Q, you should easily be able to kill him. Run circles around him while fighting to make it even harder for him to hit his Q. You cannot get hit by more than one Q or you'll die. I recommend building Riftmaker (will allow you to all-in him easier), Spirit Visage (gives you extra healing and AP defense), and a form of grievous wounds (reduce his healing he gets). I also recommend you build a QS item against him. If he ults you, he can completely waste your ultimate, take some of your health down for his team to finish you off, and if he kills you he'll take your stats and win the fight with them. You can build Zhonya's instead of QS but if he ults when after you use your ultimate you'll most likely end up losing your ultimate.

I'm not the owner of this guide but I have tons of experience with Mordekaisers and I thought I could enlighten you.

I'm not the owner of the guide, but I do know as a m7 Gwen you should always max Q first. Maxing Q first isn't for the wave, its for the damage so you can win fights. Most of Gwen's DPS comes from her Q and with her passive, more damage on her Q is more healing you'll get to keep yourself alive in fights. There's nothing wrong with maxing out E first, but maxing out Q first is just better in my opinion.

this is a top lane gwen guide yes