Depends on who writes the particular story. If she ever did exist, we would never know her full name as it is not ever recorded in history. Your best bet would be to check l'morte d' arthur. If it's not there, it's probably never been written down.
Marion Zimmer Bradley in Mists of Avalon made Guinevere's last name Pellinore.
Marion Zimmer Bradley in Mists of Avalon made Guinevere's last name Pellinore.
