I would say that if you're trying to convert Planescape's githzerai specifically rather than githzerai as they've existed in the monster books since 1st edition, drop the psionics. Githzerai and githyanki were extremely, game-breakingly powerful psionicists in 2nd edition, and it would pretty much ruin their potential as a PC race to convert that directly. None of the githzerai NPCs in Planescape were psionic, except possibly Factol Tollysalmon.
In A Guide to the Astral Plane they dropped references to inherent psionics among the githyanki, too, saying only that many of them had levels in the psionicist class.
The 3rd edition solution, where the gith races have comparatively minor psionic abilities giving them a playable +2 ECL, is another reasonable solution.
It seems like if you're using Castles & Crusades your goal is simplicity anyway, and converting all the psionic rules would be more trouble than it's worth.