So, this is my first guide... Idk what to say really, Galio is an amazing tank, best in the game in my opinion. This is just my simple build on it, I typically like to play a solo top, Galio is a good sustain at solo lane even with 2 top.
This build works bot lane as well.

Reason for Flash:

I use Flash for a few things, I use it for get aways as well as to initiate. Flashing into an initiation is good because it gets you into position to use your R (ulty)

Reason for Clarity:

I use Clarity for a only one reason, I use it to sustain in lane. With Galio's Bulwark, if you go into lane and gain creeps attention and have it on you can get full health easy. And Clarity allows you to Harass quite a lot and then taunt them with no mana.

It's not hard to figure out why I got 0/21/9, I use 21 in the Defense tree for tanking, giving me health, armor, and magic resist. The magic resist helps allow you to hit a bit harder at early game against anyone that gets hit by your Resolute Smite. Then the 9 gives you more movement and some other nicknacs that are of my own use such as when ingame with my friends he give's me blue and the Neutral buffs bonus allows you to keep it longer.

Bulwark:

This spell has a few function's, it not only heals you but also give you a small boost in Magic Resist, which can keep you in lane long as well as helps you hit an enemy harder, on many occasions you can actually use Bulwark to sustain against enemy champion attacks, such as Galio vs. Garen. Garen's basic attack is typically healed off for 1-2 hits while under Bulwarks effects.

Righteous Gust:

This spell is usually essential for get aways, nearly immediately after cast it gives you a generous speed boost allowing you to flee from most assailants.

Using Resolute Smite and Righteous Gust are very effective against killing creep, essential to both building up Warmogs when you obtain it as well as building up your CS(creep score).
But if you use these spells together properly to hit the creeps and the enemy champion(s) and force them to flee to their toward to heal or go back, keeping them from getting valuable exp and CS.

Typical runes for a tank, but being more towards Magic Resist to allow for more AP at the start to hit harder and quintessences for Health.
Not much else to say.

So, here's the good part I guess. I start with the Meki Pendent allowing me to stay in lane longer, even with Clarity you don't get to stay in lane to long, after using it you still run out of mana, but with the Pendent you tend to always get enough mana to harass whenever they move seem to be moving in for a creep kill or something.
I build a Catalyst for the bonus health and mana on level up, I wait to build the Banshee's Veil for later because Warmog take's some time to build up to it's max.
Most people wont get to the end of the line but I run a SV so that if I manage to survive a team fight I can heal up a bit faster after to either baron or just go get health faster to take down that turret.

This was me in a ranked match, I know I'm low ello, but I went 1/4/25
We had 1-2 team fights on the mid turret(s) with me tanking them mostly the entire time.
Using Bulwark on my allies to keep health up while my allies are attacked by the enemie team. I keep from dying from both there team and the turret.

This honestly doesn't have to do with this build, but before they "nerfed" Galio's ult, reducing its range and damage and such, I ran this game. Going 1v3 on a normal.