Gastron was a Time Lord soldier.
When the Doctor returned to Gallifrey and retreated to the barn where he was raised, Gastron piloted a drone to come and bring him to the Capitol. When he refused, Gastron joined the firing squad sent to the barn, along with the General and Rassilon. When Rassilon ordered them to fire, every one of them missed purposely. Gastron was the first one to lay down his gun and join the Doctor at his side, saying he was "at Skull Moon."
He was later present in extraction chamber seven when the Doctor plucked Clara Oswald from the moment of her death, and when the General was shot by the Doctor. Gastron reported that a regeneration was in progress in the chamber, and when the General emerged, now female, Gastron accompanied her and Ohila to the Matrix, where they encountered the Doctor and Clara. However, when all of their attention was on Clara, the Doctor stole a TARDIS and materialized around Clara, leaving the Time Lords in a state of confusion. (TV: Hell Bent)