Gilbert is the modern form of the Old French masculine name Gislebert, itself a development of a Continental Germanic name comprising the elements gîsal ‘hostage’ and beraht ‘light, bright’ (Forssner 1916: 115-16).
There are 11 pre-conquest entries for this name in the PASE corpus.
Forssner 1916: T. Forssner, Continental-Germanic Personal Names in England (Uppsala, 1916)
Spellings in Domesday Book: Gislebertus
Forms in modern scholarship:
von Feilitzen head forms: Gislebert
Phillimore edition: Gilbert
Alecto edition: Gilbert