Gary Clones are imitations of one of the Vault 108 originators, Gary. They are all identical and are numbered. All Gary Clones are hostile towards anything other than Gary Clones.
According to a holotape within the Vault, each Gary Clone is more aggressive than the previous.
The Gary Clones revolted against the Vault 108 residents after the scientists ran out of room to store all the Clones. They planned to kill off some of the Clones for extra room. The Gary Clones objected to this and murdered all of the residents.
Gary Clones will repeatedly say the name Gary during combat and when searching for the player. However, it is possible to say other words, such as "Damn!" or "I'm hit!"