His father was shepherd for the chief giant Ysbaddaden, and the giant had killed twenty-three of Constantine’s sons. Constantine kept Goreu, his last, hidden in a chest to protect him. When Culhwch and a party of Arthur’s knights came to Ysbaddaden’s fortress seeking Olwen, Cei (Kay) took Goreu under his wing and allowed the boy to accompany him on the subsequent adventures, in which he won the respect of Arthur’s men.
He participated in the hunt for Twrch Trwyth. Upon completion of Culhwch’s tasks, Goreu returned to Ysbaddaden and decapitated him, thus fulfilling the Ysbaddaden’s own proclamation that he must die when Olwen wed. Goreu later became one of Arthur’s warriors and advisors.
One tale states that he was a squire, given the duty of guarding Arthur’s bed. A Welsh Triad tells us that he rescued Arthur from imprisonment at the hand of Gwen Pendragon beneath the Stone of Echymeint. Goreu’s name translates as 'best', but might be a variation of Gorneu or Corneu, which is roughly Welsh for 'Cornishman'. His name was earned by him for managing to gain entrance with his followers to Wrnach's stronghold.
One curious family tree for Arthur, that prepared by T.W. Rolleston, makes Goreu the brother of Erbin and Igraine, and thus the uncle of Arthur. Furthermore, this peculiar genealogy named Goreu's father as Custennin.