Duke of Tintagel
and first husband of Igraine
According to the best known version of the legend, Gorlois was husband to Igraine, and fathered three daughters with her: Morgause, Elaine, and Morgan le Fay. His main stronghold is Tintagel, the remote Cornish castle looking out over the Irish Sea.
King Uther Pendragon fell in love with Igraine, and spends much time attempting to win her favor while playing up to Gorlois, offering him gifts. Igraine spurns him, remaining faithful to Gorlois. Finally, Uther conspired with Merlin to kill Gorlois and sleep with Igraine while disguised as Gorlois. This was accomplished through Merlin's magic, wherein Uther, Merlin, and Ulfin come to Tintagel, and Uther conceives Arthur upon Igraine.
The story--and Gorlois' name--first appears in book VIII, chapter 19, of Geoffrey of Monmouth's History of the Kings of Britain (1136). Prior to that, there seems little or no reference to a miraculous parentage for Arthur, though Uther is always mentioned as his father.
Geoffrey makes no mention of the daughters of Gorlois, but instead gives Arthur a full sister named Anne, who is married to King Lot of Orkney; this would correlate to Morgause in French romance. Meanwhile, he goes on to mention Morgan in the Vita Merlini, but it isn't clear that she is Arthur's sister. In Robert de Boron's Le Roman du Graal, it seems to imply that Gorlois has a son, but he isn't mentioned again after Gorlois's death.
Some romances, particularly in modern literature, have attempted to paint Gorlois as a brutal husband, thus softening the unflattering picture of Uther. However, the earliest romances seem to imply that Gorlois is a loving husband who deeply cares for Igraine, and that Igraine is not interested in Uther. Moreover, Gorlois is initially depicted as a brave warrior, fighting along side Ambrosius Aurelianus and acting as a counselor to Uther Pendragon during the war with the Saxons.
Would there have been a duke of Tintagel? The island--then an isthmus--has been inhabited since the days of the Romans, who built the first fort. The current ruins date to the 12th century, but archaeologists seem to agree that Tintagel has been continuously inhabited by various dukes in the Duchy of Cornwall. So while Gorlois himself seems little more than the product of legend, Tintagel itself is very real.