The "canonical" history of the people of Gith has been gradually expanded over many years, including with some changes over time. For example, the first appearance of the races was in the Field Folio, in which it was stated unambiguously that before they were enslaved by the mind flayers (illithids), the race that became the githyanki and githzerai were already evil. In most subsequent discussions, this notion seems to have been dropped.
The history of the people of Gith was first discussed in greater detail in a number of second edition AD&D works. An important source at this stage was the Monstrous Compendium Appendix: Planescape, which contains the first reference that I am aware of to Zerthimon.
The githzerai are originally an offspring of a race of humans that were freed from slavery under mind flayers by a great woman warrior named Gith. These men and women did not, however, choose to follow Gith's ways after they revolted against their slavers. Instead, they fell sway to the teachings of a powerful wizard who proclaimed himself king--and later god--of the people. The githzerai then separated from the githyanki, beginning a racial war that has endured millennia without diminishing.
Githzerai can progress as fighters, mages, or fighter/mages, and thieves. They rarely attain levels above 7th and, in any case, never progress beyond 9th. The githzerai, who worship a powerful and ancient wizard, are destroyed before they gain enough power to threaten their ruler. A major religious sect, called "zerths" (who tend to be fighter/mages) worship the memory of Zerthimon, a legendary githzerai hero, instead.
This makes clear that Zerthimon was not the original leader of the githzerai faction during the rebellion against the revolt against the mind flayers.*
A lot more detail came out a few years later. The Illithiad (1998), the second book in the Monstrous Arcana series, detailed the mind flayers. (Other Monstrous Arcana books were I, Tyrant about beholders and The Sea Devils about sahuagin. The idea of the series, which I thought was generally well done, was to give more details about the nature and culture of some powerful, intelligent, and iconic AD&D monster races. Along with the sourcebooks, there was a trilogy of adventure modules about each of the races.) Given the important foe relationship between the mind flayers and the githyanki/githzeri, there was naturally some discussion of the Gith peoples' origins. And the associated adventure modules actually had quite a bit more information about the topic; in the climactic adventure Dawn of the Overmind, the player characters actually travel to the planet where the githyanki progenitors were enslaved. Among the facts revealed in the course of the adventure are that the original leader of the revolt, Gith, was female. Gith was killed in the course of the revolt, and her two lieutenants (one male, one female) were unable to continue cooperating. They split the race into two parts, becoming the immortal rules of the githyanki and githzerai races. The man became the Great Githzerai, and the woman became the githyanki's lich queen.
However, this history was "retconned" (to the extent that it makes sense to talk about "continuity" for AD&D game rules) in Planescape: Torment, which came out only the next year. The game presents a different version of the split, with Zerthimon and Gith leading the factions. However, neither of the original revolt leaders are the current rulers of the races. This new version was treated as canonical after the game came out. (It is possible that this story may have been superseded in the nearly two decades since then, but I have not found any obvious indications of another retcon in my Web search.)
As Dungeon Master, you can chose which, if any, versions of the story you prefer. My campaigns have continued to adhere to the Ithilliad version of the history.
(*Just those two paragraphs from the Mounstrous Compendium Appendix already contain a glaring error. In the original Field Folio description of the evil githyanki, it mentions that their lich queen kills any githyanki who rise above a certain level, to prevent them from threatening her power. The description of the non-evil githzerai says something similar but not identical. The Great Githzerai prevents any of his people from reaching too high a level, rather than killing them--a neutral act, rather than an evil one. Whoever was adapting the original description for second edition evidently missed this subtlety of alignment. I mention this as a reminder that, in all this discussion, it is worthwhile to remember that the game designers responsible for these works were sometimes quite sloppy, and some of the retcons involved may have have been accidental or unwitting.)
