Galahad, the purest knight, is the son of Lancelot and Elaine of Corbenic, daughter of King Pelles, who in turn is descended from Bron the brothr in law of Joseph of Arimathea.

Like Arthur Galahad is conceived under an enchantment when Elaine of Corbenic ensorcelles Lancelot into thinking she is Guinevere. When he grows up Galahad travels to Arthur's court where is seated in the Siege Perilous. 

Galahad is one of the knights who undertake the Grail Quest and after several adventures which demonstrate his worthiness, he is one of only three knights, Bors, Percival and Galahad whoc achieve the Grail. Becoming Grail Knights and passing with the Holy Grail to the Grail Island.

"The Queste is part of the compilation called the Prose Lancelot (1215-1230), which deals with the adventures of Lancelot and his love affair with Guinevere. There are warnings in the Lancelot that his adultery with Guinevere will debar him from achieving the Grail, and the author of the Queste invented a new character as the Grail-winner, Galahad, who was 'so grounded in the love of Christ that no adventure could tempt him into sin." - Richard Cavendish, "Grail", Man, Myth & Magic, An Illustrated Encyclopaedia of the Supernatural, Vol. 9

"Like Perceval in Perlesvaus, Galaad after achieving the adventures of the Grail in the mainland castle sails out to sea to become the king of an island, whither the grail is transferred. There in the island city of Sarras, Galaad is first imprisoned then crowned." from Loomis, 'Celtic Myth and Arthurian Romance'.

Gawain = Galahad (island- Sarras) = Gwair (island- Lundy)