There is a lot of information about the githyanki hidden away in Baldur's Gate III. Here is a summary:
The artefact:
This blackened iron icosahedral artefact is an infernal puzzle box with Gith runes on its surface. This item is crucial to the main plot. As such, not only the githyanki are after this, but also several other parties and beings, perhaps even including Raphael.
Inside the artefact lies the spirit of a gith general. The general has been trapped here for a very long time, and is of major importance to the gith and others. Shadowheart was specifically tasked to steal this artefact and bring it to Baldur's Gate. During this mission, her enemies were the githyanki. The imprisoned gith general is referred to as male.
Speculation/spitballing on who the general inside is. Seeing that the gith general is male, it is likely not Gith herself, though this would have been my first guess due to the relations with Tiamat and the nature of the artefact. Who else could it be? The githyanki see this as a weapon for sure. Could it be Zerthimon (male gith general of old)? If so, his escape could rally the githzerai, which could prove a great threat to the githyanki, especially with the combined forces of Zaerith Menyar-Ag-Gith and Zerthimon combined. Mordenkainen's Tome of Foes is very clear that the githzerai are waiting for Zerthimon's return, with Menyar-Ag being the forefront of this. See this passage by Menyar-Ag: "Zerthimon does not expect us to wait in patience for his return. Instead, we must pave the way for his efforts, so that we can hasten the coming of our golden age." Are there any other male gith generals of old it could be?
The artefact is sentient (via the trapped general's spirit), and it is directly manipulating Shadowheart. It forces her to confront us, the protagonists, and our party. The general in fact specifically wants to fall into the hands of the party, with the goal to break free. This forces Shadowheart to join us, and thus she will appear at our camp even if she does not join us at the initial meeting spot, Emerald Grove, etc.
Characters and companions may kill each other over the artefact, particularly Shadowheart and Laezel.
It is possible to extract the blueprints of Shadowheart's artefact.
Some runes appear to be simple tir'su, whilst others are far more complicated. I've added two views of the artefact, one looking at it from the front, the other from the back. I've displayed each rune in the plane of your screen instead of perpendicular to the icosahedron's faces for clarity.
I noted the ones closest to tir'su as green, then the ones getting further away from it are orange, and then the ones which are really far away from tir'su are red. I only chose the runes which were closest to tir'su, but note that the ones in marked in red should not be taken as fact at all (they are just a closest guess, but I'm quite sure this isn't the Githyanki alphabet and are instead some new, unique runes made by Larian). These runes appear to just be nonsense when compared to tir'su.
The creche:
In act 1b, it is possible to access a githyanki creche, which is essentially a nursery and training ground.
From this creche, we can access a vista to act 1a (early access) or to SCL (potentially the shadow cursed lands surrounding Moonrise Towers).
There are several patrols here, and in certain cases, it it possible to be spotted and ambushed by githyanki.
There is both a dungeon and an outdoors area of the creche.
There is a "psychic extractor" located here. You will likely have a run-in with a beholder...
There is an interesting portal here, surrounded by runes. These runes essentially stand for the following letters: "A" "C" "D" "E" "H" "I" "L" "M" "O" "W"
They can be used in combination to access portal "regions":
"H O M E": Astral plane
"W I L D": Feywild (includes a potential deal with a hag)
"A C H E": Shadowfell (includes an undead ambush)
"D E A L": Avernus
"E C H O": Echoes
"H O W L": Barovia