Apparently there is no evidence of what the name was of the father of Gunnor, Senfrie, Herfast, and Avelina. Ancestral Roots states he was a forester of Arque. He was not of Crepon, but later genealogists mistakenly thought the family was of Crepon because Herbastus' son, Herfast, received lands in Crepon, probably due to the relationship of his sister with Duke Richard.
I will leave the "de Crepon" name with the family, just as I have the Plantagenet name with that family, although the Plantagenets were not known by that name until Richard, Duke of York in the 1400's.