GUINGOMAR
Gimoers, Gryngamore, Guigomar d'Avalon, Guingamuer, Guinguemar, Gwinganiers

A knight who Chrétien de Troyes calls the Lord of the Isle of Avalon. Morgan le Fay was his lover. He had a brother named Greslemuef of Finisterre (Graislemier) and, in Malory, two sisters named Lynet and Lyones. He became Gareth’s brother-in-law and companion.

He shows up at the tournament of the Castle of Maidens in Renaut de Bâgé’s Le Bel Inconnu and at the Castle Perilous tournament in Malory. In the First Continuation of Chrétien’s Perceval, his fairy lover is named Brangepart, and he has a son by her named Brangemuer. Guinguemar has his own non-Arthurian lay in which falls in love with an unnamed fairy, possibly Morgan. He may be connected with Guiomar, Morgan’s lover in the Vulgate romances.

Chrétien lists him among the vassal lords whom Arthur summoned to court for Erec's wedding.

I would guess that Chrétien's Guingomar became the Vulgate's Guiomar.