I also think, after a few playthroughs, that his jolly good mate guy demeanour might be him ingratiating himself to a good-guy PC rather than... An earnest aspect of himself so to say. Though, I admit, it might be me overanalysing things because of having played the game several times. But, here's the thing, the first time I started the game Gale was easily my favourite companion -- he's nice, friendly, and tells a good story, what's not to like? Well, later I realised one of the reasons I was so fast to favour him was that he is the only one making an effort to be your friend in a company of more-or-less hostile people. He compliments you and your actions and attempts to bond with you. And I'm no longer sure if that behaviour is how he actually is or how he pretends to be -- see, though I can't say for certain, but I believe he only acts like that when you do good things, but he also doesn't have any trouble with you doing questionable or evil things. And this might either mean that he is an neutralish guy who prefers the good but is okay with evil if it furthers his own goals... Or that he is a neutralish-to-evil kind of guy playing to your good guy sensibilities to get you to like him more.
I'm not sure how much of this is in my head. It will be fun to see where he goes.
I also don't think Shadowgirl is non-evil at all. She's just evil-but-not-without-conscience. I do think her story will develop in an opportunity to shift her morality more good-wise or affirm it in evil, perhaps forgoing her conscience.
