The mainreason why he isn't played alot in higher elos is because he has to much hardcounters that just prettymuch remove his ult, while he has most of his power packed in his ult, so if you don't lastpick him into a gargoyl-friendly enviroment, you have alot problems.
Also, most off his damage outside off his ult is focused on his Q, which is hard to land if your oponents have some experience vs gallio.
If he ever find the right setup, he can be devastating, as a good ult can win a teamfight on its own, but that rarly happens.
It's so easy to either have some on-hit cc(garren Q, Blitz E, braum passive, seriously, fuck braum...), or que up some targeted hard cc to inst cancle him the momen he flashes into the team(that flash ult is even more predictible than a flash-tibers).
Iv'e seen to many sona/blitz ults instacancle galluos ult due random timeing or fast reactions, as gallios ult has a damn casttime, so have theirs, resulting in them stating the cast right befor getting taunted, and finishing it after beeing taunted, instantly canceling your ult.
Him beeing the only meele that has no mechanic at all atatched to his AAs probably doesn't help either.
The only reason for him to go melee is because he can, and because he might actually land his Q that way.