With the semi - recent mid season mage update and all these Ap mages running around i ask myself. How tf is Galio not god tier rn. He gets a heal off of Ap dmg done to him he has a speed up to not get kited and he has an Aoe taunt that unless you have Qss or banshees youre fkd. Why is he not a reeeeaaally good pick atm
He doesn't have heal off of ap damage done to him, he has a heal based on getting hit. Big nukes only heal him once (and still hurt a lot).
He doesn't actually do that much damage if he builds tanky (even with his passive interaction with mr), plus mana can be an issue for him then. If he builds AP, then he can hurt, but he's very squishy and then trying to utilize his w can end up being counterproductive.
He just has an odd kit where his kit sortof works against itself.
There are a few reasons, the first is that he doesn't really have a clearly defined role, is he a mage or a tank? Should you play him mid or top? It's not really clear what you're supposed to do with him to people trying to pick him up. His laning phase can be somewhat volatile since he is melee and can't really rely too heavily on abilities to CS since he has mana issues. He is also a supportive type champion, people usually prefer to play champions that are all about making plays and Galio really only has a flash ult, which isn't a lot compared to some other champs.
For anyone who is interested in picking him up I'll leave a few things here on how to play him either as a mage or tank. As a mage you don't want to just stack MR, it may seem like a good idea at first but you don't get as much damage from it as you would from AP items, what you should be looking to do is get 1-2 MR items early to establish a lead over an enemy AP early then transition into building your damage items, either AP or M.Pen to take advantage of those high base damages.
If you are vs an AD champion or a heavy AD comp you still have options for your build, the first thing to point out is that Galio might be one of the best 
users in the game, since he has an AOE taunt as well as the ability to heal off of the enemy auto attacks. 
and even 
could also be considered, though be aware that use will cancel your ult.
Vs AP heavy teams 
are great early build options as a mage, but after 1-2 of these you would be better off building standard AP items, 
will give you more damage in the long run than 
vs squishier targets.
When I play Galio I usually prefer to play him as a tank rather than a mage, 
is a great early pickup as it will help you sustain in lane, with this item and your W you have a very high chance to come out ahead on trades. Rather than 
I prefer to hang on to just the and finish it into a 
later on, the rest of the build will vary but will generally be pure tank items, a full build Galio can survive for an incredibly long time in a fight and protect carries very well with the threat of his ult, slow from Q and speed buff from E, your W can give an ally an additional 90 armor and MR at 5 points in the ability increasing their survivability and still healing you.
He is actually a great pick against pretty much any AP champ. Plus he has the best waveclear in the game IMHO. He can Q,E and wipe any wave once he has a few points in both.
The problem with him is positioning for his ult to have max effect. A lot of players have trouble with that. Plus he's a melee champ usually going against ranged champs and he has to use his spells making mana an issue.
He is plenty good, the main issue is he is unforgiving in a fight. You need to land your skills and they are all small area attacks with slow travel speeds.(besides his ult)
because nobody picks him
Because things like Rek'sai, Braum, Leona, and Xin exist. There is no thing more disheartening for a Galio than having his ult canceled because of the basic nature of an enemy champion's kit. Getting KU'd/Stunned by auto attacks is devastating.
I'd rather whiff my ult than get trounced out of it again.
he doesn't stop 
or
