He was created after the Jellien that SpongeBob brought home failed to eat SpongeBob, so it ate Gary instead, while eating a plant at the same time. Unlike most other Jelliens, Jellien Gary appears for only a few seconds and is never seen again after SpongeBob leaves his house. He is only seen in the episode "Planet of the Jellyfish."
Jellien Gary is the only Jellien Clone that is a mixture of two things - a plant pot and Gary's body.
We never see Jellien Gary get separated from the flower pot, so it's safe to assume that he doesn't revert back to just Gary, or it's possible that he did, off screen.