This is the name of a character in Here Lies Arthur by Philip Reeve. A girl named Gwyna changes her name to it when she pretends to be a boy. Gwin (with an 'i') is the name of Dustfinger's (male) pinemartin in Inkheart. I thought it sounded like a girl's name when I first heard it because it is similar to Gwen, but now I quite like it.