Gormant appears twice in the list of Arthur’s men in the medieval Welsh tale Culhwch and Olwen. Gormant first appears simply as “Gormant son of Rica”, but then appears again (in the translation of Gwynn Jones and Thomas Jones) as:
Gormant son of Rica (brother to Arthur on his mother’s side, his father the chief elder of Cornwall), ....
That Arthur’s mother had a son by the chief elder of Cornwall would seem to parallel the account of Geoffrey of Monmouth’s Historia Regum Britanniae in which Arthur’s mother had as her first husband, Duke Gorlois of Cornwall. Presumably Rica could be identified with Gorlois.
However in the Welsh Bruts, the name Gorlois is rendered as Gwrleis although they are often quite willing to replace Geoffrey’s names by different Welsh names. Rica could just be another name of a minor character. Or it might be an error. There was an Icelandic king named 'Guðmondr enn Rike Eyolfson. He died in 1026. Or it might come from Africa. Geoffrey of Monmouth introduces Gormund the African into his British history, possibly from British tradition. The author of Culhwch and Olwen may have confused two or all three of these names.
In the Story of Merlin attributed to Robert de Boron, the Duke of Tintagel, who corresponds to Geoffrey’s Gorlois, is said to bring his son to court, though this son is never mentioned anywhere else in the work. According to the Prose Lancelot, the Duke of Tintagel had a son by a first wife, before he married the woman who later became mother to Arthur.
See also Gornemant who may have earlier been identical to Gormant, as in there is evidence that suggests Gornemant might have been known as Arthur’s brother.