The Gabrielideans were the natives of the Gabrielides.
Gabrielideans were a liquid-protean bioforms. (PROSE: Alien Bodies, The Book of the War) Mr Shift remembered experiencing "dripping one drop at a time into the me-pool". (PROSE: The Book of the War) However, when interacting with the rest of the universe, Gabrielideans customarily formed themselves into human skin-suits, like the one used by Mr Gabriel.
When the Time Lords allied with the Gabrielideans just before the start of the War, (PROSE: Alien Bodies) they poured a "packet of powder" into Gabrielidean soldiers to overwrite their biodata and change their physical form (PROSE: The Book of the War) so they would be able to use humanoid skinsuits and be protected from the enemy. When he spilled out of his suit, Mr Shift appeared pink. (PROSE: Alien Bodies)
The Gabrielidean military only did relatively local missions to planets like Terra Neutra before they allied with the Time Lords against their enemy in the War. After the alliance, however, they immediately began launching full-scale planetary invasions, sending battlefleets to Dronid and scouting missions to Simia KK98. (PROSE: Alien Bodies)