My theory: Glamrock Freddy doesn't kill Gregory like the other glamrocks, because Glamrock Freddy had a malfunction which he got disconnected from the main servers, which allows him to think for himself. Just like in the start of the game, after getting malfunction, when Glamrock Freddy turned on it said "SAFE MODE". And also, after scanning Gregory he said " I will notifi the main office. Huh connection error, I cannot connect to the main network."

JUST A THEORY