The gith rose up to overthrow the mind flayers that held them in servitude. But after they won their freedom, two factions among the gith disagreed on what kind of civilization they would forge. That disagreement quickly flared into open hostility, and the two groups distanced themselves from one another to pursue their separate agendas. They remain bitter enemies today, each side willing to fight to the death whenever they cross paths.
The githyanki were motivated by revenge and convinced that they deserved to take whatever they wanted from the worlds they traveled. Ranging out from the titanic city of Tu'narath on the Astral Plane, they send raiders out to plunder the Material Plane and other worlds, bringing treasures and slaves back to their ageless realm. At the same time, they hunt down and kill mind flayers whenever possible, as recompense for what the illithids did to them.
The githzerai believed that the path to an enlightened civilization lay in seclusion, not conflict. Their dedication to the principles of order is so strong that they can manipulate the stuff of chaos and use it to their benefit; thus, they have carved out a stronghold for themselves on the plane of Limbo that is virtually impervious. Though the githzerai are pacifists by nature, they share the githyanki's racial hatred for mind flayers, and from time to time they send out squads to destroy illithid outposts.
If the two races were ever to team up against the illithids, a combined force of gith could conceivably tip the balance in their favor. But as long as the githyanki and githzerai stay at each other's throats, their goal of ultimate victory over their original common enemy will likely remain unachieved.
