Heroes of the New Day
Not a lot is known about these alien peoples, except that they tend to be practitioners of psionic arts, and that they bear deep-seeded hatreds for some of their kind, as well as for the mind-flayers. Many of those found in the Prime Material plane become pirates, merchants, or mercenaries… anything to allow them to keep moving. It is rumored that great armies of Gith factions war with one another in the land between dimensions… but few are foolish enough to go there to see if these rumors are true.
Stats & Stuff
Both flavors of Gith have the same stat boosts, though with different racial encounter powers. They can also take the same racial feats, assuming they meet other prerequisites.
It’s not entirely accurate to call the Gith two races. Certainly they identify as such, but there’s no physiological reason they should be separate peoples. Instead, the divide is a spiritual and philosophical one, initially created as a byproduct of the Gith’s captivity among the mind-flayers.
Eons ago, the Gith were a slave race, and they were freely experimented upon by their illithid masters. It was found that their physiology was uniquely adept at harnessing the psionic energies of the universe, so one was bred for a greater command of controlling psionic powers, while the other was bred to be more adept at destructive psionic energies. When they finally broke free of their captors, those differences in philosophy continued to split these people. Soon hatred not unlike what you’d see in Rwanda, Yugoslavia, or [fill-in cultural shitstorm here] flared up, and the two sub-races declared war on one another.
The Zerai believed the Yanki to be militaristic and barbarous, uncivilized and incapable of mastering any “higher” mental disciplines. The Yanki believed the Zerai to be weak, elitist, and too concerned with their inner selves to have any real impact on the world around them. As with all stereotypes, there were grains of truth in each. However, those differences grew worse with time. The Yanki believed that without their physical efforts and willingness to try to change the world around them, the Gith would never have escaped slavery. The Zerai believed that without their psionic powers and their focus on the inner power they contained, the Gith would never have escaped slavery.
On top of all that, the Outlands environment that the Gith had escaped to was such an unforgiving landscape that even small disagreements could mean that thousands would go unfed as hunters refused to share food with strategists, etc etc. Then there was this Githyanki who tried to play on a Githzerai golf course, and it was all over. Okay, maybe not, but you get the idea. To save both of their sub-races from certain annihilation, the two factions separated, though there are still plenty of members of each side who still seek to do bad things to “the wrong ones” with their wrong ideas.
Of course, while they have somewhat different views now, the two factions are still the same race with the same blood. Yes, there are more Githyanki who follow a Klingon paradigm, and more Githzerai who follow a Vulcan one… but that doesn’t mean that there aren’t Githzerai warriors or Githyanki arcanists.
More detailed info can be found here