What warning does Gally give the group? What happens to him?

Gally warns the group to not to trust thomas. Ultimately, as the majority of the gladers are trying to escape the maze, when they finally get out, gally was stung, and started becoming something else. As gally tries to kill thomas, chuck jumps in front of thomas, as meno throws a spear that impales and kills gally. Although gally does kill chuck though.