Son of Eliffer and brother of Peredur and Arddun.
He is mentioned in the Annales Cambriae, which imply that he fought the battle of Arfderydd against Gwenddolau in 573. According to the Annales, this would have been 36 years after the death of Arthur, but Peredur appears later in Welsh legend as Arthur’s contemporary.
Gwrgi may be identical with Gwrgi Seferi, a huntsman mentioned in Culhwch and Olwen. The Annales say that he died in 580. The Welsh Triads add that his death took place at Caer Greu, where he and his brother Peredur fought Eda Great-Knee. Gwrgi and Peredur were both killed after their warriors abandoned them.
I got information that he is one of the cousins of the British prince Gwenddolau, the other being Peredur, who fought against the Prince at the Battle of Arthuret.
Sources
Annales Cambriae | c. 960-980
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century