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He was the son of Richard de Clare, 4th Earl of Hertford, from whom he inherited the Clare estates, from his mother, Amice Fitz Robert, the estates of Gloucester and the honour of St. Hilary, and from Rohese, an ancestor, the moiety of the Giffard estates. In June 1202, he was entrusted with the lands of Harfleur and Montrevillers.
In 1215 Gilbert and his father were two of the barons made Magna Carta sureties and championed Louis "le Dauphin" of France in the First Barons' War, fighting at Lincoln under the baronial banner. He was taken prisoner in 1217 by William Marshal, whose daughter Isabella he later married.
In 1223 he accompanied his brother-in-law, Earl Marshal in an expedition into Wales. In 1225 he was present at the confirmation of the Great Charter by Henry III. In 1228 he led an army against the Welsh, capturing Morgan Gam, who was released the next year. He then joined in an expedition to Brittany, but died on his way back to Penrose in that duchy. His body was conveyed home by way of Plymouth and Cranbourgh to Tewkesbury. His widow Isabel later married Richard Plantagenet, Earl of Cornwall & King of the Romans.
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| Name | Birth | Death | Notes
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| By Isabella Marshal, married 9 Oct 1217, Tewkesbury Abbey, Gloucester, England, b. 1160, Tewkesbury, Gloucestershire, daughter of Sir William Marshal, Earl of Pembroke, Mareschal of England, and Isabel de Clare.
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| Amicia de Clare | 27 May 1220, Usk, Monmouthshire, Wales | 30 Nov 1287 | m. Baldwin de Reviers, Earl of Devon
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| Richard de Clare | 4 Aug 1222 | 15 July 1262, near Canterbury | 2nd Earl of Gloucester and Clare
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| Isabel de Clare | 8 Nov 1226, Gloucestershire | 10 Jul 1264 | m. Sir Robert de Brus, Lord of Annandale, 12 May 1240.
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