Place of origin:
Gell guards were mostly blob-like creatures who were created by Omega, trapped in a black hole domain made of anti-matter. They had the ability to travel from Omega’s anti-matter universe into that of matter.
They were unaffected by gunfire and all the explosions and could use their claws as weapons to create explosions of their own.
Under Omega’s command they attacked UNIT HQ. They killed a number of soldiers, captured the Third Doctor and his friends, and took them to Omega. (The Three Doctors)
The Gell guards were one of the species referenced in Theoretical Monsters: A Credibility Test. (Interference – Book One)
Behind the scenes
The name “Gell guard” is never used on-screen. It first appears in the Doctor Who universe in Alien Bodies.