Guillaume de Gisors (1219-1307) was the son of Hugues III de Gisors and grandson of Jean de Gisors. According to the genealogies in the "Prieuré" documents his sister married one Jean des Plantard. They also state that Guillaume was inducted into the Order of the Ship and the Double Crescent in 1269. This order was created by Louis IX (Saint Louis) for nobles who accompanied him on the ill-fated Sixth Crusade. He has been claimed to be the third grand master (1266-1307) of the supposed Priory of Sion.
