His uncle Math ap Mathonwy, king of Gwynedd, must keep his feet in the lap of a young virgin at all times unless he is going to war. Gilfaethwy lusts after Math's current foot holder, Goewin, so to get her alone he and Gwydion steal pigs from Pryderi, king of Dyfed, sparking a conflict between the neighboring kingdoms. While his uncle is off fighting, Gilfaethwy sneaks back to Gwynedd and rapes Goewin. Math is furious when he discovers this, and punishes his nephews by transforming them into a series of paired animals using his great skill in magic, thus impressing vividly upon them the brutish ( and brutal ) nature of their transgressions. For a year Gilfaethwy becomes a hind deer and Gwydion a stag; they mate and produce an offspring which is delivered to Math. Next Math makes Gilfaethwy a boar and Gwydion a sow; when they return a year later with a son he makes them wolves. After the third year he relieves them of their punishment and makes them human again.
Gilfaethwy is a minor character in Welsh legend, and may have been used in the Fourth Branch simply to advance the story of his more illustrious brother Gwydion.
In Arthurian Romance
Like many another character in the Mabinogion, Gilfaethwy has given rise to a character or characters in Arthurian romance - in this instance Sir Griflet, who first appears as Girflet ( or Giflet ) fils Do  in the romance Erec et Enide  by the twelfth century Champénois master Chrétien de Troyes  and appears later as the eponymous hero of the romance Jaufre, the only surviving romance written in the Occitan language.
As Loomis points out, Dôn, goddess mother of Gilfaethwy, has been misconstrued to be a male character by at least as early as the time of composition of Chretien's Erec et Enide :
- Jeffrey Gantz (translator, introduction), The Mabinogion, Penguin Books, London, 1976.
- MacKillop, James, A Dictionary of Celtic Mythology pub. Oxford University Press, 1998, p.223.
- Chrétien de Troyes, 'Erec et Enide' in Chrétien de Troyes - Arthurian Romances, translated from the Old French with an introduction and notes by William W. Kibler, pub. Penguin books in series Penguin Classics, 1991, pps. 37-122.
- Loomis, Roger Sherman, Arthurian Tradition And Chrétien de Troyes pub. Columbia University Press, New York 1948, page 162.
- Arthur, Ross G., ed. (2014) . Jaufre: an Occitan Arthurian romance. New York: Routledge (Garland). ISBN 9781317693642.
- Loomis, Roger Sherman, Celtic Myth and Arthurian Romance first pub. Columbia University Press 1926 and reprinted by Constable and Company Limited 1993 ISBN 0 09 472800 3