Research Notes:
The Oxford Dictionary of National Biography states, [1]
Seagrave [Segrave], Sir Gilbert of (d. 1254), administrator and justice, was the second son of Stephen of Seagrave (d. 1241) of Seagrave, Leicestershire, and Rohesia, daughter of Thomas Despenser. His elder brother, John, having died, Gilbert succeeded to his father's estates in Leicestershire, Worcestershire, Warwickshire, Nottinghamshire, and Derbyshire on his father's death in 1241.
… [1254] on a mission with Simon de Montfort to persuade Alexander III, king of Scots, to support Henry III's Sicilian enterprise, he started home through Poitou in the company of John de Plessis, earl of Warwick, and other nobles. Despite a safe conduct from Louis IX, the party was treacherously seized by the citizens of Pons in Poitou, where Seagrave fell ill, and died in prison before 8 October. On 12 October his wardships, and the wardship of his own lands during his son's minority, were granted to the king's son Edward.
By 30 September 1231 Seagrave had married Amabilia, daughter and heir of Robert of Chalcombe; she was still living in 1282. They had two children, Nicholas of Seagrave, who became first Lord Seagrave, and Alice, wife of William Mauduit, earl of Warwick.
[1] John M. Todd, "Seagrave [Segrave], Sir Gilbert of (d. 1254)," Oxford Dictionary of National Biography (Oxford University Press, 2008, online edition), [Oxford_Dictionary_National_Biography], [OxfordDNB(UM)].