as a galio main.......
his passive is near useless. its not a "counter" to ap mid laners because it doesn't solve the issue of you getting caught out or suddenly now that you build mr items you can compete with the damage of pen/raw ap where otherwise you couldn't, he just doesn't work that way.
if you understand your mechanics and just position correctly so you dont take free damage you come out so far ahead of your enemy laner.
the secret... melee last hit, and Q every time they go to auto for an absurd amount of poke they have to take on their end. you can always safely farm with your Q + E instead of taking any damage at all.
your Q base damage is significantly higher than whatever ability they use. also your base health + regen + potential masteries far outweighs their auto attacks. you will be on the better end of trading if you land the Q
your W is so absurdly good to the point where it even stops yasuo, zed, talon in their tracks when they engage, i cant stress enough how important positioning is on galio, if you have it these guys are even a non factor. i always break even or come out ahead on galio. hes so great even that he has no bad matchups... its sad really it feels disgusting where i style on rivens and yasuos who literally cant do anything against me.
if you farm well galio is a great pick up, most people feed, find themselves levels behind, and items behind and then galio just doesn't do enough. people struggle to last hit and it really hurts them in the long run, especially on a champion they dont understand.
back at op, galio could be made meta if he simply got a ranged auto or reduced casting animations, people struggle the most with him in lane and managing his mana pool (which goes away after roa)