Gilbert is one of the “Surety Barons” along with his father Richard, named for the enforcement of the Magna Carta signed by King John of England in 1215.

GILBERT de CLARE, son of Surety Richard de Clare, and himself a Surety, was born about the year 1180. In June 1202 he was entrusted with the lands of Harfleur and Mostrevilliers. He was one of the Barons still opposing the arbitrary proceedings of the Crown. He championed Louis the Dauphin, fighting at Lincoln under the Baronial banner, and was taken prisoner by William Marshall, whose daughter he later married. He led an army against the Welsh in 1228 and captured Morgan Gam, who was released the next year. After an engagement in Brittany, he died on his return at Penros in that Duchy, 25 October 1230. His body was conveyed by way of Plymouth and Cranbourn to Tewkesbury. He was buried there before the High Altar 10 November 1230. He married Isabella, sister of William Marshall, the Surety, and daughter of William Marshall, the Protector, on 9 October 1217. She died 17 January 1239/40.

(From www.MagnaCharta.org)