One more source for Frederick (of Oosterzele):

K.S.B. Keats-Rohan, Domesday People, Boydell Press, 1999, p. 40

Here it states that by 1086 Frederick, the brother of Gerbod the Fleming (Earl of Chester) was dead and Gerbod had long since returned home, that lands in St. Omer were not held by William de Warenne. On p. 480, it connects the wife of William de Warenne, Gundreda as sister of Gerbod the Fleming, Earl of Chester. So while he is not mentioned on p. 280, taken with p. 40 it shows Frederick's connections to his brother Gerbod, and sister, Gundrada, and to his brother in law, William de Warenne. Hope this helps.

Jay