In Mordenkainen's Tome of Foes, the introduction to "Chapter 4: Gith and Their Endless War" states (p. 85; emphasis mine):
If the two races were ever to team up against the illithids, [...]
Later, under the "Githzerai" section, it says (p. 93):
The githzerai were born as a race [...]
These two quotes seem to imply that the githyanki are (lorewise) two separate races. However, under the "Vlaakith's Dilemma" subheading in the description of the githzerai, it says (p. 87):
Long gone are the days when the gith race was fully embroiled in conflict.
Are the gith actually one race with two subraces, or are they two separate races?
I would prefer 5e sources that distinctly mention this, but if none are available, then sources from other editions are acceptable.