In the legendary tales of King Arthur, Gorlois was the Duke of Cornwall and married to the beautiful Ygerna (Igraine or Ygraine).
According to Geoffrey of Monmouth, when Uther Pendragon is attracted by the beauty of Ygerna he arranges for Gorlois's death, sleeps with Ygerna under false pretences, and then marries her, becoming the father of Arthur. Later treatments such as Sir Thomas Malory's Le Morte d'Arthur keep the outlines of this story, but add magical elements, such as having the wizard Merlin cast a spell that disguises Uther Pendragon in the shape of Gorlois.
It is likely that a real Gorlois (Gwrlais in Welsh) was the governor of Cornwall under King Erbin.