This is all detailed in "Exiles From the Vault" in Dragon #298 and there's a bit more in "Underground Scenarios" in Dragon #294.
It's summarized in the canonfire wiki
The background material of the Chainmail game places the gith forerunners in a subterranean empire called Zarum in Western Oerik, where they dominated many other races from their capital city of Anithor. These gith seem to have been divided into a rigid caste system, their lives ruled by ancient ritual. The ruins of Zarum overflow with sacred spaces and temples, though the names of the ancient gith gods are unknown today. The period of Zarum's height is not entirely clear, but grey elf sages speculate it was approximately 2,000 years before the Demon Wars that ravaged Western Oerik, or 3,000 years before the present.
At some point, the illithids invaded Zarum from a neighboring plane of existence. Though the gith fought fiercely, they were no match for the psionic might of the mind flayers, and soon they were enslaved. The River of Angry Souls is a remnant of one of the terrible battles between the illithids and the soon-to-be enslaved gith. Many were brought to the Outer Planes and elsewhere to serve as illithid slaves. Other cities in Zarum were transformed into work pits where illithid overseers forced their slaves to toil for countless generations.
After Gith's rebellion, she led her people to the Astral Plane. While a few subject races and surviving illithids remained on Oerth, the gith forerunners have departed the world, seemingly for good. If they retain any interest in the ruins of Zarum, it is well concealed. A portion of the ruins of Anithor were eventually colonized by the drow of House Kilsek, who named their new settlement Kalan-G'eld.
Zarum is supposed to be the gith homeland according to this source. Other sources dispute this; Dungeon #100 names the gith homeworld as Pharagos, for example (and Pharagos is described in a way that makes it clear that this isn't Oerth). Bruce Cordell's module Dawn of the Overmind
places the ancestors of the gith on the artificial illithid world Penumbra, though they must have originally been abducted from elsewhere.
The dating (2000 years before the Demon Wars) is from Chainmail Miniatures Game: Blood and Darkness - Set 2 Guidebook
, though this doesn't fit with other things we know about gith history. The Lich-Queen Vlaakith CLVII has already ruled her people for 1,000 years, and there were 156 other queens before her since her people's rebellion. Her immediate predecessor ruled for over 500 years (according to Dragon #355).
But anyway, to answer your questions:
1. They were enslaved by illithids. After they rebelled, they retreated to the Astral Plane and Limbo and abandoned Oerth.
2. They had a whole empire; Anithor was only their capital. Another Chainmail location associated with them is the River of Angry Souls
, which is a river filled with thousands of gith ghosts.