If primary documentation exists, then Medland's hasn't yet assessed it.
To date, the introduction mentions nothing about a Crispin du Bec, father of Gilbert. Medland's Bio for Gilbert also mentions no parents:—
"1. GILBERT [I] "Crispin". A manuscript history of the Crispin family, probably written by Miles Crispin, records that the first family member was "Gislebertus" who was the first named "Crispini" because of his erect hair[1936]. Guillaume II Duke of Normandy confirmed the donation of “ecclesias...in maris Constantini pagi...” {TR: churches... in the sea of Constantine} made by “Roberto...Hunfredi...filio” to Saint-Wandrille by charter dated to [1035/53], subscribed by “Roberti filii Hunfredi, Rogeri fratris eius, Anscetilli, Willelmi filii Ricardi secundi Normannorum ducis, Rogerii filii Radulfi de Warethna, Rogerii filii Ragnulfi, Gilleberti Crispin, Willelmi Guiet”[1937].{TR: Robert the son of Hunfred, Roger his brother, Anscetille, William the son of Richard the second duke of the Normans, Roger the son of Ralph de Warethna, Roger the son of Ragnulf, Gilbert Crispin, William Guiet.}
http://fmg.ac/Projects/MedLands/enguntac.htm#_Toc157779271
Equally odd is the assumed parentage for Hawise de Guînes being Siegfried and Elstrude. Medlands shows the couple with one child: ARDOLF de Guines (posthumously 966-)... &c. —https://fmg.ac/Projects/MedLands/NORTHERN%20FRANCE.htm#SiegfriedGui...
Yes, it's possible that primary sourcing for other children of Siegfried and Elstrude hasn't come to the attention of Cawley and his colleagues. Nonetheless, there seems to be more guesstimation thank scholarly source identifying going on at this juncture.
