Gaea is post-apocalyptic Earth in the distant future. There is no indication as to how many years into the future it is, but all signs would indicate it's very far away, as most of mankind's structures have been completely destroyed and most of the planet has been grown-over with dense jungle.
Beast Wars II cartoon
While going through transwarp space, Lio Convoy and the Maximals, as well as Galvatron and the Predacons, landed on Gaea. Galvatron was drawn to Gaea due to the high levels of the powerful energy source known as Angolmois riddled throughout its surface. Lio Convoy, aware that the universe would be doomed if the Predacons got control of the Angolmois, stayed with the Maximals to fight them off.
While the Maximals chose bodies based on the local wildlife, the Predacons selected bodies based on the war machines of the planet's former inhabitants.
Eventually, Galvatron used his battle ship, the Nemesis, to suck all the Angolmois out of the planet. Thanks to the intervening of Lio Convoy, the Angolmois was sealed away in capsules and scattered across the cosmos before Galvatron could use it.
Beast Wars Neo cartoon
After the disappearance of Lio Convoy and the Maximals, the Maximal Elders sent Big Convoy and his new recruits to investigate Gaea. They found Lio Convoy's black box, learned about the Angolmois capsules and set off to retrieve them.
- It is never specifically stated in the show itself that Gaea is Earth in the distant future. Instead, this was only made clear many years after Beast Wars II, in Lio Convoy's Robot Masters bio.