It's funny because I love Astarion and I just can't relate to Gale.
I think Gale is probably not one of the neutral companions either, at least I hope that he is actually evil because maybe then he would be a little interesting. As things are, he just bores me to death.
And thats with the bomb inside him and him having banged a deity. There is something about his voice that puts me to sleep and the entire character is just academic, sterile, and yes... boring.
And that's fine, I think he is probably the type of person who acts all casual and unassuming before he turn to stab you in the back.
Everyone would expect Astarion to betray you, but not Gale, which is why he'd make a better villain IF it turned out that there is something haflway interesting about him. And when I say interesting, I mean personal.

Astarion on the other hand is just bursting with his selfish manipulative personality. His charm is clearly a way of telling us that we ought to forgive and like him because he is so quirky and witty, and it actually works! There is a deep undercurrent of desperation to him that I hope we will get to explore, and I am honestly curious where they will take him. The guy is a vampire, a condition without a cure, he is also evil even though his character understands that there is no satisfaction that will fulfill him forever. He also wants to survive and be free at the same time, and he is torn between these two, without wanting to choose one or the other. His character is compelling because it's relatable.