Gally is a Glader who did not trust Thomas from the beginning, turning into Thomas's first enemy in the Glade.
In The Maze Runner, Thomas described him as a fifteen year old who was tall and skinny with black hair, green eyes, and a nose that was the size of a small fist that resembled a deformed potato. His voice was described as being scratchy.
He was said to be named after Galileo. He is the main antagonist of Maze Runner and the anti-hero/supporting protagonist of Maze Runner: The Death Cure.
He was portrayed by Will Poulter in his first villainous role.
The Maze Runner
According to Winston, Gally would have recovered a part of his memories after having been stung by a Griever in the middle of the day near the West door. Gally became a Glader after arriving in the Glade. However, he was despised by most of the Gladers due to his arrogant nature. Two years later, Gally witnessed the arrival of a teenage boy named Thomas who Gally immediately became enemies with.
After being interrogated with Thomas by Newt, Minho got into an argument with Gally that ended with Gally running off into the Maze and going missing. After missing for many days, Gally eventually returned to the Glade, scared and dirty, and warned the Gladers that the Maze's creators were going to kill them and that Grievers will take one of them every night. Plagued by a nameless terror, he snatches planks barricading windows and hit Newt with one of them. Thus a Griever manages to penetrate in the farm and seizes Gally before taking him in the Labyrinth.
Gally was apprehended by WICKED and taken into their control. When the surviving Gladers made it through the Griever hole, Gally emerged from the shadows with a knife and while under control of WICKED, attempted to kill Thomas. A young Glader named Chuck jumped in front of the knife and was killed in Thomas's place. Thomas, angered by Chuck's demise, attacked Gally. While Gally and Thomas fought, a group of adults entered the chamber and killed the WICKED agents. Thomas knocked Gally unconscious and fled with Teresa and the other Gladers.
In the film, Minho presumably killed Gally with a spear, so his appearance in The Death Cure in the films is questionable. However, Gally was shown breathing after the spear wound. Also, his voice actor was confirmed to appear in the film, meaning it's highly likely that he survived, although how he could survive after a spear wound is unknown.
The Death Cure
Unknown to Thomas and the other Gladers, Gally was taken into custody by WICKED agents and imprisoned in WICKED's headquarters. During his imprisonment, Gally felt guilty for Chuck's death as he could not control it and made several attempts to escape, but was stopped by the WICKED guards every time. However, a group of spies snuck into the facility. They told Gally that if he acted crazy WICKED would get rid of him and put him in a city somewhere in the United States where Gally and the spies could meet again and Gally could join them.
He later sends a message to Thomas, Minho, Newt, Brenda and Jorge to meet them at his apartment in Denver, where he now works for the Right Arm trying to take over WICKED. He explains to them how he escaped and tells them how WICKED is after them and Hans, the man they need to get rid of the implants in their brains. He also tells them that Teresa and the rest have been there, and if they wanted to join the Right Arm after finding them, they should come see him.
When Thomas and Brenda are led later on to the leader of the Right Arm, Gally is with him, and he assured him that Thomas is to be trusted. He becomes part of the "army" that takes the Immune back to WICKED to take over it, but then ends up joining Thomas and the rest after they discover the Right Arm's true intentions of blowing the place up, leaving the Immunes in the Maze.
He helps Thomas rescue the rest of the Immunes, going back to the Maze and then fighting against the Grievers and Janson's men before going through the Flat Trans to the last paradise with the Immunes.