It seems very unlikely to me that
Galahad could be related to
Gilead. In this list of the names linked to below of characters in Arthurian legend, the only two that are from the Old Testament are Lot and
Joseph -- and the latter isn't really a living character in the tales, it's just that the Biblical character
Joseph of Arimathea gets
mentioned in Arthurian legend as the person who supposedly brought the Holy Grail to England. It seems likely to me that the equation of
Galahad with
Gilead has been done by someone who wrongly thinks that a superficial similarity in sound or spelling is enough to claim that one name is a "variant" of another.
http://www.geocities.com/CapitolHill/4186/Arthur/htmlpages/kingarthurlegendpeople.html