Don't worry: Gally won't be throwing any knives in this book. He's actually on the good team.
Gally was the kid in the first book who hated Thomas's guts from the very beginning, went cuckoo bananas and ran out of the Maze only to end up killing Chuck at the end. Basically, Gally is everyone's most hated character.
But it turns out that WICKED used Gally, too (surprise): they manipulated him and forced him to do all of that crazy stuff, and he's suffered immensely from the psychological pain. In The Death Cure, in fact, Gally has quite the opening line: "'Glad you came,' Gally said in his raspy voice. 'Because the end of the world is upon us'" (24.51).
Huh, seems like an upbeat kid.
Gally eventually reveals to Thomas that he acted really crazy back in WICKED's headquarters so that he would be released by them. When he was finally released, he joined an underground group called the Right Arm, which is bent on spoiling WICKED's plans. So Gally actually ends up being a good guy, after all; he just got a super unlucky role back in the Maze.
(Wait, didn't Teresa also just get an unlucky role back in the Maze? Oh, well.)
We can really see Gally's true character when Thomas asks him to follow the Gladers instead of staying with the Right Arm. When Thomas asks, we get this description: "He saw a sadness in the boy's eyes that made his heart break. And he also saw something he'd never seen there before: trust. Genuine trust" (66.29).
We get the sense that all Gally wants is to be loved, or at least to have friends. These hopes were taken away from him by WICKED back in the Maze, and he suffers greatly because of it. Hopefully we can forgive Gally for killing the innocent boy, Chuck, back in Book One; Gally was just as much a puppet for WICKED as anyone else.