Governor Gumpas is one of the main antagonists in the novel and film of The Voyage of the Dawn Treader by CS Lewis. He is a traitor to the Narnian crown.
Governor Gumpas was assigned the lordship of the Lone Islands by Caspian himself and was originally paying tribute to Narnia. However sometime the tribute stopped coming and Gumpas was soon a traitor to Narnia. Gumpas had taken to trading slaves and selling them to Calormen, a disagreeable and warlike nation to the South. Calormen had always been trying to invade Narnia and this was insult to injury.
When Caspian landed on the Lone Islands, he met the Lord Rhince, who had fallen in love with a girl there, and had settled, but he hated Gumpas and his slavery. Lord Rhince was one of the Telmarine lords who had been betrayed by Miraz, and so Caspian tried to undo the slavery. Caspian and Lord Rhince went to the office of Gumpas and Caspian more or less said "I'm the King. Stop this nonsense," and Gumpas was revealed to be a cowardly fool who couldn't even do proper business. He was stripped from his position and made a duke, sent back in exile and made never to deal with slaves again or any Calormen merchants.
In the film of the Voyage of the Dawn Treader Gumpas was played by David Vallon and was far more evil than his novel counterpart. He did not simply deal slaves, he sacrificed them to an evil Green Mist which apparently devoured them. For this act, Gumpas crossed the Moral Event Horizon into a cruel tyrant. Gumpas was very warlike and entered any fight arrogantly. Unlike his novel counterpart, he was not a man to simply sit down and do business. He openly fought Caspian and wanted to kill him and his crew, and he captured Caspian, about to sacrifice him to the Mist. However, Caspian's crew came to find him, fighting Gumpas and his men. Gumpas ordered his soldiers to kill the attackers and he battled them all but was killed by Eustace of all people, who killed him by accident.