Githyanki Ecology and Physiology (GAP)

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The githyanki resemble the humans that they descended from in the most basic ways only. While still humanoid, their rough, leathery skin is pale yellow, and their features are gaunt in the extreme. Fact is, some githyanki could be described as skeletal in appearance. Their black eyes've sunk far back into their long, angular skulls, and their noses've receded to become very small and highly placed, giving their entire face the appearance of having been stretched vertically. Raven-black or fire-red hair adorns their heads and is usually worn to reveal their pronounced, pointed ears which are often marked by ridges gained in dark ceremonies.
The dark of it is that while they were illithid slaves, the githyanki (and githzerai) underwent horrible experiments by their masters in an attempt to make them great warriors for the mind flayer empire. They were also forced to work in areas no human was ever meant to be, where strange energies altered them for all time. Lastly, those who would one day become githyanki altered themselves with their own incredible resolve and hatred, twisting themselves in mind, body, and spirit.
As a result, githyanki have an inherent magical nature. Each can travel the planes at will, using an ability most nongithyanki call plane shift, and the githyanki call jez'rathki, meaning literally "seeping through the cracks." They're also capable of other magical skills not available to humans or other races (see below).
'Course, most githyanki exhibit psionic talents as well. As with humans, all attacks, defenses, disciplines, sciences, and devotions are available. These abilities give a wide gamut of powers to the githyanki, shaping their outlook, tactics, and strength.
A githyanki's lifespan is approximately twice that of a human's. However, since they do not age on the Astral Plane (where time does not pass), githyanki lives usually end up lasting much longer (to the perceptions of those involved). The githyanki have developed a spell so that time does not catch up with them when they leave their home plane. If a githyanki does not have access to such magic, he may be loath to leave the Astral, especially if he is old, because of the instant aging that would occur. Those that frequent other planes do not worry as much since they haven't cheated time as much as other githyanki. 
Like everyone else, githyanki don't need to eat on the Astral Plane. For their frequent off-plane missions and adventures, however, they usually take food and supplies with them. Although omnivorous, the githyanki digestive system (which has atrophied slightly due to long periods of dormancy) requires frequent repasts of meat. Hunters sometimes travel to other planes to obtain meat, although astral whales're also common prey. If necessary (that is, if they are desperate), the githyanki'll eat fallen foes, except that they'll never partake of the foul flesh of illithids, and they are not cannibals, so they also refuse the flesh of githzerai and other githyanki. Farmers called g"lathk raise fungi and other plants that don't require sunlight within githyanki cities and fortresses.
The most pronounced deviation of the githyanki from their human origins is that they have become egg-layers. This is either the result of some experiment of the illithids or the exposure to some strange astral energies. In either case, this method of reproduction closely resembles that of reptilian creatures. Within githyanki communities, there is always an egg chamber (or possibly many of them), where all eggs are kept and guarded. Caretakers raise the young in a communal fashion - there are no githyanki families.
Because of the lack of actual time passage on the Astral, caretakers (known as varsh) of eggs and young, must plane shift the objects of their attention to another plane (usually the Prime) periodically so that they can mature. Because of this hassle, and because githyanki live so long, the birth rate is usually fairly low. Once beyond the very young stage, githyanki trainees are usually forced to spend a great deal of their time on the Prime or another plane, both for the experience and so that they will finish physically maturing in a temporal environment.