Before his rework, Galio support was kind of a niche pick, only really picked if the Galio was experienced, kind of like old Galio in general, his lack of a gap closer made him a slightly underwhelming support, only really able to use his flash when it was up to get kills, but his rework changed all of that.

When Galio's rework rolled around, Galio was meant to be a vanguard played in top lane, but lots of players found different ways to play him really quick, wether it be Galio jungle, AP Galio mid, or what my personal preference is, Galio support, today I will try my best to take you through Galio support and how strong it can really be.

My experiences with Galio didn't start for a while, before his rework, I only first played him when he was given to me in an Aram game, and overall, it was fairly enjoyable, so I saved the Ip, then bought him. Immediately I figured out why not very many people played him, his kit was extremely boring to work with, only really useful with his ultimate, even then, he had no gap closer outside his flash and maybe protobelt to get into range for his devastating ultimate, so I shelved him pretty quick.

Then his rework came along, at first glance, I didn't really care much for his rework, I mean I didn't despise his rework, but compared to the most previous rework, (Warwick) It felt pretty sub-par to the wolfman. When his rework hit live servers, I decided to give it a chance and play one game with it in top, I was shocked with what I found, It was a blast, I immediately took him to the Jungle, it was fun there aswell, quickly Galio joined my many M5's, then I did something a little odd for me, I took him support.

Originally, I didn't really think support Galio would be that strong, but when I took it up, I saw how much he could dominate other Melee supports, I immediately started to play it more and more, eventually raising Galio to the rank of M7, Right now, Galio is one of my main supports, and I don't plan on dropping him any time soon.

Armour seals and Magic resistance marks are standard for most tanks, they also work well with your resistance stacking.
Magic resistance quints are mainly use for your passive and defensive spells, they give extra power to them which can help you out early on.
Health per level marks are a must have since your build lacks health, this gives you that little bit of extra health to tank more damage.

Cunning:
Wanderer- Helpful when it comes to roaming, since you won't be last hitting often, you don't need to take savagery.
Secret stash-You aren't in a solo lane so assassin is useless, and you aren't jungleing so runic affinity is also useless. Secret stash helps with your sustain in lane from poke and other form of damage by providing a bigger heal.
Meditation-You burn through mana fairly quickly fairly early, so meditation helps you keep your mana in check so you don't have to worry about grabbing early mana.
Bandit- Standerd for most supports, helps you get some extra gold when you go in for a passive auto and also when your carry takes farm.

Resolve:
Unyielding-The extra armour can be helpful, but what we're focused on is the extra magic resist. The little bit of extra magic resistance this mastery gives you can make an impact on your defensive spells and passive, you may not see it, but I assure you, the benefits are there.
Tough skin- This mastery is helpful for more aggressive carries in the lane phase, the reduced damage can make life quite a bit easier when you go in for relic shield procs.
Runic armour-You can have up to four shields, you have your magic shield, the shield from relic shield, your Courage of the colossus shield, and your locket shield if you decide to get a locket, so runic armour makes all those shields stronger.
Fearless-Galio likes to stack resistances, Fearless gives quite a big raise in resistances when you make your engage, this is helpful for tanking any damage that is thrown at you right away.
Legendary guardian- Similar to Fearless and Unyielding, This mastery gives you extra resistances, you take it for the same reasons as the other two.
Courage of the colossus- This mastery is great for Galio, it gives you a shield when you land any of your three hard cc's (Ult, W, E), and that shield is usually for quite the large amount, which really helps you be the frontline you're supposed to be.

Relic shield- Simple reasons behind this, you're a melee tank support, relic shield is meant for these types of supports, and also it synergies very well with you're passive.

Core items:
Eye of the equinox- This item combines your sightstone with your support item, the raw stats aren't that astounding compared to face of the mountain, but the reason we buy this item is to free up an item slot since most of your core builds don't have many active items, more on active items builds when we discuss optional items.
Abyssal Mask- This item works wonders for Galio, it gives him some nice tanky stats with the MR and health it offers, and the passive really helps your mages deal more damage since you should be in the middle of the enemy team when you play Galio, thus, triggering the passive on most enemies.
Merc treads- This is also simple, you need the tenacity so you don't get cc'd to death when you engage a fight, the Mr is also helpful for your passive, W and Ult.
Iceborn gauntlet- This item may seem odd but just hear me out on this. Iceborns passive slow is what we are focused on here, it offers great lockdown potential by slowing targets to help keep them in a zone, this allows your carries to get kills much easier since targets have a harder time escaping. Its also worth noting the extra damage it gives your autos makes your passive auto do a nice extra chunk of damage, which can sometimes be super beneficial when it comes to picking fights. The mana is also a must have since you burn through mana fairly quickly early and mid game.
Vs Ad items:
Deadmans- This item is fairly similar to Iceborn, it gives your passive auto more damage, while making you more tanky, it allows you to chase targets down to help your immobile carries lock them down.
Knights vow- While you may argue there is better support options out there, this item offers a fair bit more armour compared to most support items, while giving off health. The passive portion can help you peel for your carries by directing damage they take to you, which in certain cases may mean the difference between their life or death.
Vs Ap items:
Banshees veil- Now since this is a mage item now, it may seem a bit odd to take this on a tank support, but hear me out. This item adds a surprising amount of damage to your kit, while giving lots of Mr to you in the process for your defensive spells an your passive, this can help you pick off stragglers and chunk down enemies when you decide to all in.
Locket- This item still gives you a little bit of armour for any potential ad champions on the opposing team, but it still gives a nice 60 mr for you and your spells, the active is also very strong, the shield value is pretty high and can sometimes be the deciding factor in a fight.
Protect & Peel-
Redemption- This item may be targeted towards supports with healing after its change, that dosent mean its still a decent item to grab if you want extra peel, that being said, its very situational, only get it if you are in desperate need of peel within your comp.
Optionals-
Ruby sightstone & Face of the mountain- If you prefer grabbing lots of active items, lets say, locket, redemption and stoneplate, consider going for a ruby sighstone & face of the mountain over eye of the equinox, since ruby sightstone lowers the cooldown of most of your items, which can be really helpful in coming fights, Face of the mountain is only got here since you still need a support item.
Gargoyle stoneplate- This item is a very niche item for support Galio, the extra resonates it provides is helpful with your passive and defensive spells, but most of the time, your health isn't that over the top, which makes the active seem kindof lacklustre, only get this if you want those extra resistances and have enough health fromm your other items to make actual use out of the active.
Protobelt- This item is a very rare item to buy on support Galio, only buy it if you want an extra gap closer if you feel the enemy team is too far out of reach from your e to get a good w, if you are already fine with landing the e+w's, stay away from this item.

Summoner spells:
Flash- Standard for most champions.

Exhaust- Standard for most supports, in Galio's case, it is helpful for early fights by reducing the damage the opposing carry can do to you, just make sure you avoid using it too early or too late, if you use it after your carry as already been chunked down, it will end up being wasted. Later in the game, almost always use this on their carries, the brief window of reduced damage they deal can greatly impact the outcome of a fight.

Ignite- A very rare case, only take this if you are 110% certain you can dominate your lane, with this, look for a super early engage, almost always as soon as you hit level three.

Champion spells:
Order: R<<Q<<W<<E

Q) Winds of War: This is Galio's main form of harass, its damage as a support is very deceiving, it still can pack quite the punch if multiple ticks are landed of the tornado part. While you max it first since its your primary source of damage, you always use it last in your combo, this is since you can use your taunt to land all the ticks on a single target, usually their marksman. The only thing you need to be careful of its to make it count whoever you use it, it has such a high mana cost that it burns up your mana quick, even with an iceborn, try to use it only when you know you can atleast land two ticks.

W) Shield of Durand: The Passive on this spell makes Galio a huge problem for mage supports, he can tank much of their damage, add an abyssal and a locket to that and mage supports should have a huge problem dealing with you. The active is mainly used after you've used your E to get closer and then smacked them with an empowered auto, try to channel it for a little bit to get an increased taunt duration, the damage reduction makes you able to stay in for longer and tank more hits for your carry.

E) Justice punch: This is what old Galio lacked; a gap closer. This spell is great for making engages, even if you miss it, you can set yourself up for a good taunt, hitting it procs CoC, making you even tankier and even harder to take out. You also get to use your passive empowered auto right away, which actually can hurt quite a bit. Just remember this may seem to make a good escape, but the dash back at the start can make fleeing with it extremely challenging, try to line it up so that you don't get behind enemies with the dash back.

R) Hero's entrance: This is a great spell for roaming and protection, you can use it on your tanks or assassins when they make an engage to help them soak up some extra damage while providing a beautiful knock up if they are able to cc some of their team, its also a good way to burn their summoners if they are scared of the knock up. Another use of this ultimate is if your backline is being engaged on, if one of your carries gets caught by an assassin or tank, you can opt to ult them to try to scare off to knockup any of the attackers.

Pros:
Amazing all In potential.
Good poke.
Fairly tanky for a support.
Great counter engage tools.
Great into mage supports.
Cons:
Runs out of mana fast.
Taunt is Telegraphed.
Very vulnerable after all spells have been used.
Most adc's ignore his magic shield.

Early game: Levels 1/2, all you should be doing is poking, use your Q to chunk down your laners while you prepare for level 3, one you hit level three, look at their health, if either of them is below 50% max hp, which they should be if you landed most of your q's, go for an all in, if they aren't below the quota, look at the matchup, if they have something like a Thresh, continue to poke, if its an easier matchup, try the all in.

If you get engaged on pre level three, use your taunt or knockup to counter their engage, get your passive auto off, then your Q, this should chunk them down quite a bit, if they still want to fight, don't go for it, your cooldowns are too high to win the fight, just back out, and continue to poke.

Mid game: You should be looking to roam as much as possible, if you see mid being ganked, ult your laner, or even if your laner is going for a tower dive the damage reduction is very beneficial to try to keep them alive.

If a skirmish breaks out, Use your ult one of two ways. One, Ult a squishy if they are getting engage on by a tank or assassin, or two, ult a tank or assassin engaging, the first way of using your ult dosent really utilize the knockup so much as the damage reduction, however it can be very beneficial to keep your carry in the fight, in contrast, when you ult a tank or assassin thats engaging, the damage reduction is not as helpful, but the knockup can be devastating, getting a good 3 man knockup can quickly turn the tide of a fight.

Late game: You should now only be ulting your carry when they get engaged on, keeping them alive is much more important then getting the knockup, always keep your eye on your carry during any fights, taunt anyone who tries to go in on them and use your E on them right after, this should give your carry time to reposition and burst the opposing team.

If your carry ends up getting killed, look for someone else to protect, if all you carries are dead, get, out, of, that, fight, the ONLY time you should be staying in a fight is if there is one or two enemies that are low, just take note of their build, make sure they can't out sustain you, if you get an ace in this situation, take an objective, your pushing power isn't super strong, but you should be able to get a tower or two with the long death timers.