One of my all time favorite sets of entries from the Fiend Folio was for the Githyanki and Githzerai. I received my copy in 1982, and was mesmerized by the amazing artwork of Russ Nicholson. A few years later, while in college, I won a costume contest for my Githyanki Cosplay. I had the double shoulder pads, the straps of embellished armor, a funky two handed sword and a pointy hatted, tasselled, mask I had cast out of latex. Alas, no pictures remain, the costume was destroyed and the mask stolen.
I had tried building them as a playable 3e race, but lost that manuscript and many others when my storage unit flooded out. I thought of them again when I found the Astral Reaver entry in Dwimmermount, and Of course, while looking through DF14:Psi. Now that book contains an entry for Chaos Monks, which have a little of Githzerai about them, but I find that while cool and definitely creepy, they lack the serene insanity of that group of escaped Elder Thing slaves.
These builds will follow character construction. I am afraid that I may have to write up both a 125 point base mentalist template, and a 125 point lens to go with it at some point, probably calling them Sensitive and Pychic; the Psychic lens would be the same as the 50 point Mentalist lens, with IQ +1  Psi Talent +2  and 85 points of further mentalist advantages, powers and skills. (Update: this already exists asthe 125pt lens Psi in DF15 )
The races of Gith are Jumpers, and contrary to what Things Psi Can’t Do, on page 7 of DF14, it is a psionic ability. The power has a limited number of uses per day, about three, so the total cost is 80 points (Jumper, can trace +10%, limited usage -20%, Psi powered -10%)
These have the following traits: Jumper(as above), Elder Talent 1 Intolerance[-10], bloodlust [-10], Chummy[-5] (because they are always found in groups).
Base templates (from DF15 ) would be Skirmisher (with sword and weapon mastery) or Squire (without lance, and all riding and animal handling skills focused on Slorn) plus the 50 point Mentalist lens.
The Reavers have a variety of leader types,, some adding the 50 point Knight/Wizard lens, others with the full 125 point Adept lens. They also have unholy warriors (plus mentalist lens), and may have 50 to 75 points of mentalist abilities. Their upper echelon will have Veteran+Knight(with weapon master:Sword)+Psi.
They are believed to be ruled by a Lich (see Pyramid 3/72) + Priest+Adept+Psi.
The reavers usually wear heavy Lamellar armor, and all of their equipment is ornate. They tend to have quite a bit of magic, leaning toward armor enhancements and Silvered swords with Ghost Weapon.
These are a little different, and not as inclined to be magical.
Their basic traits include Jumper as above, Magic Resistance +5 and unfazeable, Will +1 vow of silence[-10] 
Their base template is the Skirmisher (martial artist option) +Mentalist lens.
Their leaders are full Mentalists, +Monk lens, or Skirmisher +Mentalist lens + Martial Artist/Wizard Lens (with an upgrade to their magic resistance to enhanced magic resistance), or Veteran+Skirmisher+Mentalist or Psi.
Their leader is also a Lich, but built as a Mentalist +Adept+Monk+ Skeletal Undead.
Void Mystics use plain and simple equipment, and tend toward light armor, leather or thieves’ mail, and may have magical or silvered swords, but tend toward short swords, while the Astral Reavers are fond of two handed and bastard swords. Void mystics get along with Void Brutes.