Final answer:
Gally believes that Thomas is a spy due to his suspicious behavior and association with WICKED.
Step-by-step explanation:
In the Maze Runner series, Gally is initially suspicious of Thomas because he arrives in the Glade with no memory and seems to have a secret motive. Gally also witnesses Thomas having conversations with other Gladers that raise suspicions about his intentions. For example, Thomas talks with Chuck about reuniting with WICKED, the organization responsible for the Maze Trials. Gally's distrust of Thomas is further fueled by the fact that WICKED is known for conducting secretive and unethical experiments. All of these justifications contribute to Gally's belief that Thomas is a spy.