Galio Support is a fairly niche pick that ends up being incredibly effective into certain matchups. He's got fairly good engage, outplay potential, and forces the enemy into playing a certain way to avoid your ult and disruptive nature.

PROS + Strong engage potential, able to taunt a whole team + Naturally tanky + Surprising damage potential + Very powerful ult with lots of range + Extra strong against teams with high magic damage Galio Support is able to act as a very powerful guardian for his team, helping his ADC obtain kills on one end of the map before ulting to save his jungler brawling on the other. His mere existence makes the enemy play differently as he may show up to assist a teammate and turn a fight at any time.

CONS - Slow - Reactable - Team-Reliant Because of this, Galio therefore relies on his team. He can't Hero's Entrance onto himself meaning, if he wants to get any mileage out of it, he needs a teammate to target. He also isn't super effective at doing damage alone especially because all of his abilities have a noticeable start up to them.

	Flash: Nearly all champions need Flash as it is the most flexible summoner spell in the game. Galio used to be able to Flash during his Shield of Durand but sadly he no longer has this combo. He can still flash first before casting the ability though it gives the enemy more time to react.
	Exhaust: A safer pick that scales well into the game. Ideal against burst or a particularly fed opponent, alongside having a slow to help setup kills.
	Ignite: A summoner spell best used for lanes you believe you are going to win as it allows you to finish off opponents after an all-in.

There are two main rune choices for Galio Support, one for a massive speed-boost, and one for general tankiness.

RESOLVE

We take the Resolve tree because there are many runes that enhance Galio's survivability. Aftershock effectively allows Galio to taunt multiple targets, or attempt to charge directly into the enemy team and still survive, making it easily the best keystone for Galio. Guardian isn't a terrible choice, especially because Hero's Entrance actually readies Guardian on the target on-cast meaning you can potentially save a teammate's life somewhat like a Shen.

Font of Life provides additional healing to your allies for anyone you CC. Galio is particularly good at CCing multiple targets at a time, helping your team just a little bit during any fight. Shield Bash is also not a terrible choice due to Galio's passive naturally giving him a shield, but it usually falls short of Font of Life.

A choice between Conditioning or Bone Plating. In general, Bone Plating is better in lane while Conditioning aids in Galio's tankiness outside of lane.

Overgrowth is ideal for late game in order to add to your bulkiness. Unflinching is also a good choice for Tenacity whereas the additional healing/shielding with Revitalize isn't terrible, but doesn't add up to much over the course of a game.

Perfect Timing pairs nicely with Gargoyle Stoneplate and the free Stopwatch allows you to survive certain plays you may not otherwise, such as Hero's Entrance-ing into the middle of an enemy team or to aid in a tower dive and living. Biscuit Delivery is ideal for sustain in lane with additional mana as Galio has the tendency of being poked down and running out of mana. He can optionally sell the biscuits for additional gold.

Galio's shards are very flexible. In general, he should bring shards depending on the primary damage type of his lane matchup.

INSPIRATION

We take the Inspiration tree because it provides the best tools to help Galio engage in and out of lane. Prototype: Omnistone is an incredibly unpopular Keystone which revolves around randomness. Now for a Support Galio, your major keystone is not all that important. The focus of the Inspiration tree is bringing Hextech Flashtraption anyway, but Prototype: Omnistone is not even a bad keystone and, instead, requires you to play around what you get. It's also not entirely random as you are more likely to get certain keystones depending on the situation. Unsealed Spellbook, on the other hand, is objectively less random than Prototype: Omnistone but also a lot less fun.... That being said, the ability to choose what summoner spell to use is strong throughout the game.

The general opinion on Hexflash is usually pretty mixed. Some do not like it, believing it to be fairly useless and essentially baiting you into making bad engages whenever it's off cooldown. Others see its usefulness, whether it's enabling you to take creative flank and ganks, or helping you escape bad situations. For someone like Galio, you can easily Hexflash over a wall from out of vision and taunt a whole team, especially aiding in the fact Galio cannot punch through walls.

Biscuit Delivery helps with your sustain in lane, essentially functioning as free potions. If for some reason you don't find a use for them, you can even sell them for a bit of additional gold.

Cosmic Insight is a general rune of deceptive usefulness. It reduces the cooldown of your abilities, your items, even your summoners. Due to this, Galio can make more frequent uses of his abilities, and even helps with reducing the cooldown of items such as Gargoyle Stoneplate.

Nimbus Cloak represents the second part of this two part combo involving Hexflash and Nimbus Cloak. The speedboost you get from Nimbus Cloak is equivalent to the one you get from smite even though Hexflash has a substantially shorter cooldown. Celerity applies to nearly everything you have, including your Q, Nimbus Cloak, your items. The speed boost it provides is fairly substantial.

The shards Galio can bring are fairly flexible. In general, you want to bring defensive shards depending on what support/adc combination you are facing.

Galio Support is very fun and effective. Consider trying him on occasion.

Thanks to @JHOIJHOI's exceptional guide on making guides which can be found here: Making A Guide

This guide is heavily based off of my Volibear guide. You can check that out too.