Gale death is tragic and undeserved for a character who's as innocent as a meth dealer can be. He's taken in the middle of a chess game that went all over his head.

But one thing I wonder about him is: how would he react, knowing Gus is about to kill Walt. And does he actually guess it or not? I feel like he understands that something is off when Gus asks him how many cooks he needs to be able cooking Walt's formula by himself. Gus tells him Walt has cancer but I'm not sure of how much he buys it, but he remains passive in that situation, and doesn’t try to learn more, he's just shown embarassed wheb with Walt, but won't cross any line about it, and ends up accepting that one more cook will do it, while obviously Walt is not in such a bad shape that he'd be one cook from dying.

My guess is that he preferes to remain in ignorance. That passivity may be the one fatal flaw tarnishing his "innocent" nature (still meth business hence the quotes). He actually dies in that ignorance, maybe not even recognizing who's Jesse.

I think that's also why Walt would favor Jesse over him, because Jesse is the opposite of willing to ignore things and remain passive. If he suspects that Tomas was killed by Gus dealers, he'll investigate and take action about it. He's ready to take extreme actions to protect the ones he feels owing a protection to (like Tomas, even if it backfires, and like Walt who's threatened after rescuing by him). Jesse becomes a murderer (which is huge for him) out of not wanting Walt to be killed in the afternath. He confronts Gus at any veiled threat he'd make ("If you kill M.White you're gonna have to kill me too"). I feel like Gale wouldn't put himself in the line to defend Walt, and I think that's a part of why Jesse is so important as a partner to Walt, compared to Gale.

Gale is skilled, nice and well mannered (and just to tell it once more: undeserving of his fate), but I'd say his personal sin (as every character has his flaws) is his will to ignore any harm involved in the business he's in. Be it the murder in preparation right next to him, or even the way he sells something he would never take for himself, as he's not an addict and probably never saw meth for other than it is, but it's none of his business (ironically) as long as people are willing to buy. Still among the nicest meth dealer we get to see.