Firstly, I will point out that I am only really familiar with D&D 5e and the video game Neverwinter Nights 2 (which is based on D&D 3.5e and set in the Forgotten Realms; incidentally, if the answer to this question would be different for different settings, assume Forgotten Realms).
In that video game (since the githyanki feature quite heavily, certainly in the first third of the game), one of the characters (Aldanon the Sage) claims that the gith were once human before the illithids enslaved them (see this gameplay video, 4:13):
Ages ago, the ancestors of the githyanki were human, and inhabited another plane of existence, where they were enslaved by the illithids, or mind flayers.
However, this is just a video game character, not a source book or similar, so this information is hardly credible. But this information may have come from somewhere credible, since I only know of the official information from D&D 5e.
Furthermore, there is also this D&D Beyond video with Mike Mearls, where he also claims that the gith were once human (around 5:55), but I'd prefer to have that confirmed in a source book rather than just a passing comment by Mike Mearls (especially since it seems to contradict the D&D 5e Monster Manual, at the very least; see below).
However, the above contradicts what D&D 5e officially has to say on the matter; from the Monster Manual (p. 158):
Before there were githyanki or githzerai, these creatures were a single race enslaved by the mind flayers. […] Whether these tall, gaunt creatures were peaceful or savage, cultured or primitive before the mind flayers enslaved and changed them, none can say. Not even the original name of their race remains from that distant time.
This seems to heavily imply that what the gith were before the illithids enslaved them is simply not known. Perhaps they were human? Perhaps they weren't? The point is, we don't know (meaning, there is no canon information).
I believe this is the most information we have about the origin of the gith in D&D 5e (since, although there's a whole chapter on the gith in Mordenkainen's Tome of Foes, it doesn't seem to talk about much before their uprising against the illithids), so I'm hoping that earlier editions of D&D may have more information (according to the gith tag information, they go back as far as AD&D 1e, so there should be lots of lore to go on).
There is also this quote from the Forgotten Realms wiki's page on the gith:
Some sages claimed that they were once descended from humans,[5][1][13][14] while others maintained that their original race was in fact unknown.[2][15][16]
and although it does include references, I don't own most of those books and therefore cannot verify them; also the quote itself is vague (i.e. "some sages claimed", which I assume is a way to rationalise in-universe why these two conflicting pieces of information exist).
Is there any canon information from any edition of D&D to support the claim that the gith were once human before the illithids enslaved them?
Best Answer
Githyanki/Githzerai were described as originally human when first introduced
The githyanki and githzerai were first officially introduced to AD&D by the 1st edition Fiend Folio sourcebook (developed from an entry for White Dwarf magazine's Fiend Factory section). In that manual there is a brief description of the history of the race in the githyanki section:
And the githzerai entry immediately following notes that the githzerai are another offshoot of this race that diverged from the githyanki.
This history and Gith society was significantly expanded on in the 2e sourcebook A Guide to the Astral Plane for the Planescape setting, which restates that the illithid were overthrown by the rebellion of their once-human slaves, who had been "twisted in form" by generations of slavery and mind flayer influence:
In this history, the Gith were definitely originally human, and their races are named after the individual who successfully led their rebellion against the mind flayers. Sourcebooks for later editions of D&D have become more circumspect and ambiguous concerning the origins of the Gith races, but as far as I know none of them have explicitly contradicted this story - it's one of the bits of D&D lore that has been remarkably consistent across editions and settings.
Of course, as it is described as being ambiguous/unknown in-setting in modern material it is up to any given writer/DM whether or not they consider the older, more definitive statements to be canonical - the matter is at least consistently described as being so far in the past it's entirely reasonable that no mortal historian could possibly be sure of the truth of it. It seems like Mike Mearls and the writers for NWN at least still consider it relevant, though.