It’s an unending war between two cosmic races… who are exactly the same. You may tell me that the githyanki are chaotic evil raiders from the Astral Plane, totally different from the lawful neutral githzerai monks of Limbo, but just how different are they?
1. The githyanki and the githzerai are both descended from the slaves of mind flayers. (They parted ways soon after they freed themselves.)
2. Both the githyanki and the githzerai are focussed on hunting down the remaining mind flayers.
3. Mind flayers are about the only thing they hate more than each other.
4. Both races have psionic abilities that they received from their former masters.
5. Each of the gith races is ruled by an ancient, powerful figure – an ancestral hero from the war against the mind flayers.
6. In both cases, the leader is so ancient that it’s not entirely clear whether they are alive or dead.
7. In both cases, the leader’s immense age means that they’re dependent on their followers.
8. Both races expect the return of a long-departed hero – a kind of gith Jesus.
9. Both believe in a promise of paradise in the afterlife.
If you are using the gith races in your adventure, you should do everything you can to show how ridiculously similar they are to each other, but how much they hate each other all the same.
One of the ways that the two races are differentiated in Mordenkainen’s Tome of Foes is in the way their leaders are portrayed. Both are depicted straddling the boundary between life and death. It’s suggested that Vlaakith, tyrant of the githyanki, is actually consuming the souls of those who’ve ‘ascended’ to paradise. Menyar-Ag, leader of the githzerai, is portrayed as an ancient master whose psionic strength keeps him vital as his decrepit body withers away. He’s dependent on his disciples to care for his physical body. I would suggest finding ways to give the impression that Menyar-Ag is no different to his githyanki counterpart, and that he may also be consuming the souls of his followers. (It doesn’t have to be solid evidence, just enough to make your players suspicious.)
This would also muddy up the alignment of the githzerai, suggesting that their leader, if not the race in general, is actually evil. You could also suggest that their lawful nature is just a facade. Maybe their practice of stabilising Limbo is a metaphor for the stabilisation of their own chaotic nature? Every now and then, you could have a githzerai’s calm exterior crack, revealing the chaos that lies within.
Think of these two races as sects of the same religion, who have a violent disagreement about how many celestials can dance on the head of a pin. It’s a darkly comical way of saying that we may be just the same as the people who go to war against.
If you want to learn more about the gith, pick up a copy of Mordenkainen’s Tome of Foes.