Gil's Planet is the home planet of Gil Marverde, it was destroyed by Silent Scavengers roughly 15 years before the story's beginning. Its name and relative location are unknown.
When Gil shows his follower identification to the guards in Titan headquarters, his race is listed as "unknown." Assuming his race was native to his home planet, it is possible it has something to do with why it was targeted by scavengers, and why Titan's army appeared as quickly as they did.
The idea of this environment was that it mirrored the complete freedom of Gil's childhood.
Gil is saved from this paradise only to be raised to work towards living in one he has never seen.