One of the central tensions in D&D's official worldbuilding efforts is the way that the desire to create the most Approachable Vanilla Fantasy collides with the desire to create Unique Recognizable Branding. If you look at where we've been so far, so you can see this in action. D&D is the inheritor of Tolkien's fantasy races, but over the years the various stewards of the D&D brand have attempted to stamp those races with marketable product identity to sets them apart from what other fantasy rpgs are presenting. We have the usual trifecta of dwarves, elves, and halflings--but with the additional, particular flavorings of Corellon, Moradin, duergar, shadar-kai, drow, tinker gnomes, Garl Glittergold and Yondala and all the rest.
The gith sidestep D&D's connections to generic fantasy and insert an element that is particular and Very D&D. The gith, like many other beings in the multiverse, are former slaves of the mind flayers. The gith rose up against their squid-faced masters, threw off their shackles, and...promptly split into two factions who hate each other: the githyanki and the githzerai. The githyanki and githzerai are the D&D version of the Romulan/Vulcan split.
You can tell that the githyanki are the bad guys because they're down with slavery. The githzerai are secluded monks seeking enlightenment. As usual, the bad guys are the more interesting of the pair: the githyanki rove out from a city built atop a dead god (!!!), serve a lich-queen who sits upon a throne made of mind flayer skulls (!!!), and ride dragons (!!!). The githyanki live at the intersection of heavy metal album cover and sci-fantasy paperback book cover and are all the more awesome because of it.
Vlaakith, the githyanki's lich-queen, is shady as fuck. Back when the gith were rebelling against the mind flayers, she encouraged Gith--the leader of the rebellion who granted a name to the gith people--to venture down into the Nine Hells to strike a deal with Tiamat for a little red dragon back-up. Gith never returned from the Hells, but the gith did get some dragon allies. Once the mind flayers were defeated, Vlaakith declared that the githyanki would pick up where the mind flayers had left off: taking slaves and plundering the Material Plane as they saw fit.
Vlaakith is also running a scam on her own people. She has spread the idea that her best and most loyal warriors will be rewarded by admission into a Special Paradise. As part of the preparation for this righteous ascension, there is a private ritual...in which Vlaakith sucks out their souls and gains their powers, Highlander style. This is not the 100 Virgins they were promised.
Oh, and Gith's fate? Nobody knows. Arcane pellcasters who try to look into it trigger a curse that turns them into allips--insane, undead shadow-ghost things. Anyone who tries to look into it with divine magic just experiences a fucked up cosmic void: "Those who try experience a strange sensation, as if their minds were teetering on the edge of a great abyss, one that spans time, space, and memory."
Githyanki are hatched from eggs! And they are raised in a militaristic society where they are forced to fight each other until only the strong survive--so there is a touch of Evil Sparta to their culture. Vlaakith doesn't understand the importance of cannon fodder, apparently. When they aren't fighting and raiding, the githyanki are languid and decadent--a bit Melnibonean, actually. Vlaakith has to invent shit for them to do just to keep them occupied--she literally sends them on scavenger hunts because left to their own devices the githyanki are people who can't manage to finish anything they start.
Githyanki have spelljammer ships, but I don't think they're ever actually called spelljammers in the text even though they reference the helms and the need for spellcasting pilots. Why didn't they use the word spelljammer?
There are no bars in Tu'narath, the githyanki city. Bummer.
The githzerai live in a terrible neighborhood. They are deeply lawful beings who live in Limbo--a plane so chaotic that they have to constantly use their mental powers just to keep the environment at bay. The githzerai were led into this property blunder by Menyar-Ag, who is now a bit like the Emperor in Warhammer 40k: a decrepit but powerful leader who can't do anything physically but is still alive mentally. Also, similar to the 40k psykers who keep the navigational beacons going, there are githzerai anarchs who harness the psychic powers of their people to keep Limbo from consuming their communities. The githzerai also have a space marine analog in the zerth--githzerai "chosen ones" who await the return of
Life in Limbo, for the githzerai, takes place in monastery fortresses drifting through the plane. When the githzerai venture to other planes, they take adamantine citadels with them. They have missionaries who like to recruit psions to the philosophy of Zerthimon: "Have you heard the good news?" And there is a gith reunification movement. Good luck with that one.
The emergent theme of Mordenkainen's Tome is that conflict divides people and short-circuits their ability to get shit done. If the githyanki and githzerai joined forces, they could really stick it to the mind flayers, but they're too busy fighting each other to exterminate the illithids.