With the info we have, Gale definately reaches for the Good rather than Neutral. Unless you have some strong argument regarding his actions that I haven't noticed.
I don't know about strong argument but in my opinion, the approval system doesn't indicate what the companion would do, but simply what that they approve or disapprove of.
So, Gale might appreciate that our Tav is a kind soul and doing good things without striving to do the same himself.
He didn't accept being spurned by Mystra. What he did afterward was pretty self-serving. He didn't want to loose her attention. He wanted to keep basking in her glory and love. So he did something big, dangerous and stupid.
I mean, don't get me wrong, I like Gale and I don't think he's a villain for what he did. He is a relatable human character. But I wouldn't really color him good...not just yet. That being said, if he's Neutral, I do agree that Gale has a stronger tendency toward good. He even recognized that what he did was wrong.
But hey, at the end of the day, alignements are not just based on actions but also the "true" intention behind them (you did "good", "nothing" or "bad"...but why?). So, it can be tricky to put people in boxes based on that.
