An Arthurian knight, the son of Do. In one version he, not Bedivere, was given the task of flinging Excalibur into the waters after Arthur's last battle. When he saw Arthur's tomb he became a hermit but he died shortly afterwards. As Gwydion seems to have been a British smith-god, Gilfaethwy was presumeably also a deity. Griflet's father, Don, seems to come from Don, the goddess who was Gilfaethwy's mother in British tradition.
Accounts vary as to how it came into Gawain's possession.
He and Gawain went to the Isle of Man where, after some adventures, they slew the king and Gromer, restored to his original shape by decapitation, became king in his place. He demanded that Arthur discovered what it was that women most desired. Arthur was told by the Loathly Lady, Ragnell, who demanded marriage with Gawain as her reward. Gromer's name means 'Lord of the Summer's Day'. He is analogous to Hafgan in the story of Pwyll, and bears many similarities with Gawain's other adversary, the Green Knight.