Although he is decidedly insane, no doubt as a result of the Master's experiments with psychic powers in humans, Gideon is still the most comprehensible of the psykers, and the only one who can provide any real information. Gideon is very loyal to the Master, albeit in a rather delusional manner, and speaks very highly of him and his mission in his discussion with the Vault Dweller. He is described as a normal-looking human male, fitted with a psychic nullifier.
Gideon was a receiving-telepathic-dominant psyker with minor photo-kinetic abilities (though this ability is never viewed in-game), and did not possess the ability to control his telepathy, which meant he required the psychic nullifier to block incoming thoughts. This would explain Gideon's link with the living walls in the Master's Lair.
Destroy the Mutant leader: Gideon informs the Vault Dweller that it is the minds of the Master's failed experiments that inhabit the living walls of the Lair, and that he shares with them a telepathic link. He also informs the Vault Dweller that the only way to suppress the walls' psychic attacks is by wearing a psychic nullifier, which he can be convinced to provide.