Commentary
Cockyne (VII, page 672) says "Gilbert, father of Walter, is said to have been 2nd son of Ralph de Gant, Lord of Alost, circa 1038-1056 (A. Duchesne, Maison de Gand, p.111). Phair and Sherman agree with Cockyne.

Weis places him as the son of Rudolph, Lord of Aalst, which is probably that same as Ralph.

Charles Cawley: "A manuscript genealogy of the Gant family names “Giselbertus de Gaunt, filius Baldwini comitis de Flandria” when recording that he accompanied “Willielmo conquestore avunculo suo” to England. Flemish primary sources do not confirm that Gilbert was the son of Baudouin Count of Flanders. However, it would be chronological consistent if he was the same person as Giselbert, son of Rudolf van Gent, although the primary source which confirms that this is correct has not yet been identified. "

Charles Cawley places him as the son of Ralph de Gent and Gisela de Luxembourg.

Charles Cawley does not list a daughter Agnes

G.A. Moriarty, 'The ancestry of Gilbert de Gant', The American Genealogist, 34:39-40 (1958).
Family
ClaimDetailEvidence
FatherRalph de Gant, Lord of Alost (e995->1057) [S756] [S775:VII.672] [S3202]
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MotherGisela de Luxembourg (e996-) [S756] [S3202]
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SpouseAlix de Montfort Dame de Montfort-sur-Risle (e1043-) 
Child +Walter de Gaunt (e1083-1139) [S775:VII.672] [S925] [S3153] [S3202]
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Attributes
ClaimDetailEvidence
GenderMale
NameGilbert de Gant [S756] [S775:VII.672]
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NameGilbert de Gaunt [S756]
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NameGilbert de Ghent [S756]
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NameGilbert van Gent [S905] [S2611]
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NameGilbert de Gand [S3153]
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NameGiselbert van Gent [S3202]
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FatherRalph de Gant, Lord of Alost (e995->1057) [S756] [S775:VII.672] [S3202]
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MotherGisela de Luxembourg (e996-) [S756] [S3202]
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TitleLord of Folkingham [S905]
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Timeline
ClaimDateDetailAgeEvidence
Birthest 1029 (1023-1036)
 
Living1056 [S3202]
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Flourished1066 [S756]
probably arrived in England
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Military1068Yorkshire, England [S756] [S775:VII.672]
Commander in York
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Military1069Yorkshire, England [S756] [S775:VII.672]
was taken prisoner by the Danes
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Flourished1086Lincolnshire, England [S756]
was tenant-in-chief and one of the largest landholders
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Flourished1086 [S775:VII.672]
was a tenant-in-chief and one of the largest landholders in Lincolnshire
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Deathabt 1095 [S756] [S775:VII.672] [S905] [S3202]
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Sources
IndexTitle
[S756] Weis, Frederick Lewis, "Ancestral Roots of Sixty Colonists Who Came To New England before 1700, 7th Ed." (Genealogical Publishing Co., Inc., Baltimore, 1999).
[S775] George Cokayne, "Complete Peerage" (https://dcms.lds.org/delivery/DeliveryManagerServlet?dps_pid=IE83409).
[S905] Cawley, Charles, "Foundation of Medieval Genealogy (Medieval Lands - Normandy: Alencon, Evereux, Meulan, Perche)" (2006-7 (v1.1 Updated 02 March 2007)) (https://fmg.ac/Projects/MedLands/NORMANDY NOBILITY.htm).
[S2611] Cawley, Charles, "Foundation of Medieval Genealogy (Medieval Lands - Normandy: Aumale, Rouen, Eu )" (v4.2 Updated 30 May 2018) (https://fmg.ac/Projects/MedLands/normacre.htm).
[S3153] Cawley, Charles, "Foundation of Medieval Genealogy (Medieval Lands - Brittany - Cornouaille, Penthievre )" (v4.3 Updated 17 Apr 2023) (https://fmg.ac/Projects/MedLands/brittcope.htm).
[S3202] Cawley, Charles, "Foundation of Medieval Genealogy (Medieval Lands - Flanders: Nobility)" (https://fmg.ac/Projects/MedLands/FLEMISH NOBILITY.htm).