Gantok was a humanoid who worked for the Silence. When the Doctor asked for the Silence's 'weakest link', he was directed to Gantok. He played live chess, which he made sure he had protection against by wearing a gauntlet. A proud individual, he could not stand losing a match to anyone, planning to kill whoever beat him; even if it was the Doctor, who was already going to die at a fixed point in time.
Gantok wore sterotypical Viking-style attire, along with the Eye Drive worn by many servants of the Silence. He was an avid player of live chess. He was proud of his skill at the game, and when the Doctor had beaten him during a match, Gantok offered to pay anything to escape forfeiting or electrocution.
He told the Doctor that Dorium Maldovar knew many things about his death, despite being met with disbelief; Dorium had been beheaded at the Battle of Demons Run by the Headless Monks. Gantok helped him get into the Seventh Transept, where he turned on the Doctor, angry at him for beating him at Live Chess. Despite the Doctor trying to warn him, Gantok was killed when he stepped on a booby trap that dropped him into a pit filled with the skulls from the Headless Monks, which proceeded to devour him. (TV: The Wedding of River Song)
Behind the scenes
Counting spitfire pilot "Danny Boy", for whom Gatiss provided the voice only, Gantok is the third character to be played by Doctor Who writer Mark Gatiss on television, the other being Professor Richard Lazarus. Mark Gatiss is not credited in The Wedding of River Song under his own name, but as Rondo Haxton, a reference to actor Rondo Hatton, on whom Gantok's acromegalic facial features seem to be based.