Galaas, Galachin, Galaishin, Galaschin, Galasshin, Galathin, Galescalain, Galescin, Galeschalains, Galeschalians, Galeschin, Galeshyn, Galessin
Sommer in a footnote makes him the son of King Nentres of Garlot (and thus the son of Elaine of Tintagil and a nephew of Arthur), but points out that the Vulgate in other places makes him the son of King Alain of Escavalon (and thus the brother of Floree) or the son of the King of Norgales (North Wales) and thus the brother of Dodinas le Savage.
Another suggestion says he is Arthur’s nephew, named as the son of King Nentres of Garlot and Blasine (Arthur’s half-sister) in the Vulgate Merlin, and as the son of the king of Escavalon in the Vulgate Lancelot. Although his father rebelled against Arthur, Galescalain dreamed of becoming one of Arthur’s knights. He deserted his father and joined with the young Gawaine, his cousin.
Further on, Sommer explains that Galeshin and Dodinal are cousins, not brothers. Also in the Vulgate, Galeshin is identified as the Duke of Clarence, a title he was given by Arthur; “Galeshin” certainly could be a variant of the name Malory uses for the Duke of Clarence, “Chaleins“. Take your choice. Galeshin would seem to be a most flexible character.
Galeshin and Gawain headed for Arthur’s court, fighting several battles against the Saxons along the way, in which they were assisted by Merlin. Arthur knighted Galescalain for his brave service, and he became one of the Queen’s Knights, and later Knight of the Round Table.
Arthur later made him the duke of Clarence, the city he had protected against the Saxons. This is an anachronism: the duchy of Clarence was created in 1362 and the place name to which it related was Clare (Suffolk), a small woll town. He fought in the war against King Claudas. Later, while on a quest to rescue Gawaine from Caradoc of the Dolorous Tower, he saved the Lady of Cabrion and liberated the castle of Pintadol.
In other adventures, he was imprisoned in the Valley of No Return, the Dolorous Tower, and the Forbidden Hill (Le Tertre Deuee), but was rescued from all three by Lancelot.
Sources
Vulgate Lancelot | 1215-1230
Vulgate Merlin | 1220-1235
Le Livre d’Artus | Early 13th century