He inherited Brionne from his father, and was assassinated about 1039/1040.
145Guardian of the young orphan, Duke William of Normandy, 'the Conqueror'.
152About 1015 two illegitimate sons of Duke Richard I, Godfrey and William, became counts, the latter being count of Eu; and after their deaths, Gilbert of Brionne, the son of Godfrey, was a count, whilst William's son, Robert, was before 1047 recognized as count of Eu.
152When Count William of Eu died shortly before 1040, Gilbert did assume the land and title, but he was assassinated in 1040 and his young sons, RICHARD and Baldwin were forced to flee Normandy, finding safety at the court of BALDWIN V, COUNT OF FLANDERS. When WILLIAM THE CONQUEROR married COUNT BALDWIN's daughter, he restored Gilbert's sons to Normandy, although he did not invest them with either Brionne or Eu or a comital title. WILLIAM granted the lordships of Bienfaite and Orbec to RICHARD FITZ GILBERT, and Le Sap and Meules to Baldwin. Although Gilbert's descendants later pressed a claim for Brionne, it was never restored. RICHARD and Baldwin ftizGilbert took part in the Norman conquest of England, and both assumed important positions in THE CONQUEROR's reign.
--- Michael Altschul *A Baronial Family in Medieval England: The Clares,1217-1314*, Baltimore MD (Johns Hopkins Press) 1965, p 18-19