If you read older sources, the older name for Galahad is “Galaad.” That’s the Latin Vulgate spelling of “Gilead.” Sometimes you see people adding an extra H for their pronunciation convenience, and that probably would be where the H in Galahad came from.
Of course, Gilead probably came in as a Latin/English/Frenchification of his original name, Gwalchavad, which is super-Welsh and would be way too hard to say if you weren’t from Wales or Brittany. Defaulting to a nice Biblical name would have been a good compromise. It may have led to Galahad becoming more saintly of character, too.
Anyway, I’ve known this for a while, but it’s not something obvious or well-known, so I thought I’d throw it up there for the Internet’s sake.