The Greys were grinders within the Church of the Outsiders. They had LEDs put under their skin and had voice changers implanted into them. Some also implanted electric stun guns. They believed that the other Outsiders were pathetic and not following the true path.
The Greys were suspicious of Ianto Jones and planned to do him harm after he asked to join them. However, they were paid a visit by Jack Harkness who had them release him. They infiltrated the Hub with Andromeda Ross after chasing Gwen Cooper and were subdued by Gwen using a Belitrusca stun wand whilst inspecting Ianto's trouser press. (AUDIO: Believe)