The Gabrielideans were a liquid-protean lifeform.
Before being converted into an idea-based lifeform by the Enemy, the Shift was a Gabrielidean soldier fighting for the Time Lords against the Enemy. (PROSE: Alien Bodies)
Behind the scenes
In his commentary for Alien Bodies, author Lawrence Miles reveals that he intended them to be connected to the Droge of Gabrielides, who the Fourth Doctor said had offered an entire solar system for his capture in TV: The Sun Makers. [1]