TheIn Maze Runner, the group escaping the maze is able to open the accessway to the control room through 2 things -- holding the amputated leg/electronic "key" taken from the Griever and knowing the 8 digit combination based on the cycle of sectors exposed. As Gally is a keeper and not a runner, he wouldn't necessarily know the exact order of the sectors the way Minho does -- in fact, he shouldn't even have been able to find the area behind sector 7 because he doesn't run the maze. And when he does, he doesn't have the electronic key held by Chuck which opens the doors which slammed shut as our heroes opened the inner coded door.
So how did he get there?