I think beeing melee is fine for gallio.
The thin feeling ankward is that he gets no reward from going melee. Any other melee champ has some mechanic conected to AAs in some way, or is expected to get items that empower AAs, only gallio doesn't.
What he lacks is any reason to actually go melee besides the ult.
I think he should loose his current passive and instead get some bonus damge on his AAs and some better base stats. His base ad+as are rather weak compared to most other tanks.
He's somewhat similar to chogath, he can poke/waveclear from afar and needs to land his skillshots to set up plays while haveing some great potential if he gets to the right spot.
His Q is not only poke, the slow also allows to close in on the victim, same with E.
E is easy to land, so you want to open with it, then you use speedboost to get in a good position for Q, and then you can close in on the slowed target(s) to....do something...this is where his design fails. His E and Q can work great as soft-engage tools with an ranged aoe slow and a teamwide speedbuff, but gallio him self has no reason to use those tools to get into close combat unless it's to ult.