Dr. Gilbert M. Sullivan was a famous television host and relationship specialist who had a secret passion for hunting. In addition to the hunting of animals, Sullivan had a special prey, the peaceful aliens known as the Manicoids. When Gilbert was just a boy his southern gentleman of a father shot down the first Manicoid ship to come to Earth. His father was killed when the ship crashed into him. Dr. Gill managed to salvage the Manicoid teleporter ray and used it to teleport Manicoids to his private hunting reserve where he would hunt them down, kill them, sever their heads as trophies, and, in most cases, send the body back to where it came from.
Dr. Gill was killed by the Manicoids after the Middleman used Wendy Watson's middle-watch to lead the Manicoids to the Doctor's private hunting reserve. ("The Manicoid Teleportation Conundrum")