I assume it's because Gale's approval is so easy to get if you're playing a character even remotely good-aligned (as most folks do), he's quite friendly/funny, and he's conventionally attractive. Casual gamers looking to romance a man will probably find themselves on his route without effort.
Astarion on the other hand is a nightmare to get approval with if you don't want to be a prick to everyone, so people had to really go out of their way to romance him. Meanwhile Wyll just doesn't have enough content. You don't even really know anything's going on until he suddenly wants to dance with you in act 2.
For the ladies, Shadowheart is really easy to get approval with with the right dialogue choices but also easy to piss off. Karlach is the most straight forward on a good run and Lae'zel approval kicks off hard in the Creche if you just let her do her thing.
Fun fact. You can persuade Gale into foursome with drow twins and Halsin will still insert himself in there and you cant poly romance Gale and Halsin(Gale rejects it). Not sure about approval/disapproval thing in that situation, but, correct me if im wrong, Gale will actually disapprove if you reject Halsin in foursome xd.
Far as I know, you can convince him to tag along anyway and he'll just watch or something? Poor Gale. It's disgusting, really, but at least it's the player pushing for it.