As quoted earlier, from the Guide to the Astral Plane:
What actually occurred at that meeting is one of the best-kept secrets in all the planes. Simply put, no one knows but Gith, Tiamat, and apparently one of Tiamat's consorts, a red great worm named Ephelomon. What is known is that Ephelomon came to the Astral and instructed Vlaakith that Gith had named the wizardess as the githyanki leader's successor, and that she was to carry on her legacy of conquest, for Gith would not be returning. The great worm also announced that an eternal pact between red dragons and the githyanki was now in place.
It's critical to any discussion of the outcome of Vlaakith CLVII's destruction to go back to these strange events which crowned Vlaakith I as Gith's successor.
What of Tiamat? Was Gith the victim of duplicity on the part of the dragon? While that's entirely possible, the subservience of the red dragons for millennia afterwards would seem to indicate an honest bargain where both sides gave something valuable in order to receive something they considered equally valuable. Yes, this does help the dragons a bit, but it helps the githyanki considerably more. Particularly since the dragons *serve* the githyanki. Also, recall that in "The Lich-Queen's Beloved", the destruction of the Staff of Ephelomon immediately renders the pact with red dragons null and void, so the bargain is more than mere words. The githyanki lost Gith, and received the incredibly valuable resource of red dragons to aid them in their conquests. What, exactly, was the price Gith paid them for this ongoing service?
What of Gith? Destroyed or not? The idea of Gith somehow voluntarily sacrificing herself to Tiamat somehow rings false. She would be depriving her people of her galvanizing leadership, while merely gaining an alliance. Perhaps she did so in exchange for the promise that Tiamat would do whatever necessary to prevent the githyanki from ever suffering a wide-scale genocide or enslavement? Any of these things could be true, but it doesn't really add up.
Was Gith's disappearance a well-crafted plot by Vlaakith to seize the throne? If so, it was an elaborate one. Again, note the existence of the Staff of Ephelomon and the service of red dragons. The pact with Tiamat in fact exists. If it was Vlaakith that traded with the dragons, what did she have to offer? The head of her leader, while perhaps of interest to Tiamat, would have hardly been adequate payment for thousands of years of dragon service. The red dragons that cooperate with githyanki receive easy loot and conquest, and that is of some benefit to Tiamat's long-term plans to spread the influence of evil dragons. Is it the githyanki who actually serve the dragons, in a way? Again, the fact that the dragons stop supporting the githyanki when the Staff is destroyed would indicate that they would prefer to not assist the githyanki if they were not bound by Tiamat's agreement.
Perhaps Gith went on some important mission for Tiamat, performing a task that the dragon found impossible to do without her assistance. It must have been an epic task to merit an open-ended promise of aid from red dragons from that point onward for thousands of years. What could it have been? We know that a "short time" after Gith met Tiamat, Ephelomon comes with the message of the pact. So had Gith already died attempting to do something for Tiamat, or was she merely unable to return? Either way, she succeeded at acquiring the aid of the dragons, so in at least that respect, her mission was a success.
So without knowing how or why Gith disappeared, there is no way to know if she might return, either in her original physical form, or as one reincarnated with the spirit of Gith. Is it possible that her pact with Tiamat also included the provision that Tiamat arrange for her return should her people ever need her? The so-called gods certainly won't be provided that service to Gith's heathen people.
The githyanki have no doubt discussed all these scenarios, and many more. And I'm sure the theories have become increasingly elaborate in the eons since Gith first left on that ill-fated errand. The points above are merely a starting point for a more detailed discussion of the possible scenarios of those events, and the ways in which they might impact the current drastic events that are even now shaping the next era in githyanki history.
The successor of Gith is gone, with no heir. What will become of the throne of Gith if it is simply taken by the most powerful and influential githyanki? What will become of the traitors who committed blasphemy by arranging for the attack on Vlaakith? What will these events do to the well-established and strict boundaries of Githyanki culture and society?
And what will happen when the githyanki equivalents of religion and prophecy, come into play? When githyanki realize that they are once again in a time of monumental change, a time of future legends?
Thoughts, planewalkers??
It must be remembered that, ultimately, the githyanki (and githzerai) are "human", in the loosest sense of the term. Whatever bargain was struck between Gith and Vlaakith I and Tiamat, included obviously Gith's absence for whatever reason. I suppose that Gith herself was reasonably powerful, so she might have become one of Tiamat's favored servant in Baator, or some such. True, the "deal" for red dragon assistance (to characterize it as servitude is excessive, and some sources specifically speak of it as an equal partnership) seems to favor the githyanki, but god-only-knows what Gith herself had to offer. Vlaakith seems to be a secondary figure. Perhaps Tiamat makes the deal in the hope that (or expectation based on knowledge of the future) Gith herself, now in the "care" of Tiamat, will/may someday return to the Githyanki and bring the whole of that race (and maybe the human race as well) into "the fold", in a Power-going-for-the-big-money sort of way. Face it, humanity is the ultimate worshiper-base on the Prime. There is no race so successful or so widespread as humans. Githyanki, as a human off-shoot, are a major prize. In the long-term, multiversally speaking, they might be the only prize that matters. And if the gith races can be instrumental in the development of humanity as a base of divine power, then Tiamat may already be in a unique position to "corner the market", under certain circumstances. I mean, really, most of Western mythology is based, one way or another, on the story of the Assyrian god Marduk making the world out of the body of Tiamat..................... Tiamat's "revenge", so to speak, might be the coopting of the human race for her own ends. I digress, therefore I stop.