Gilbert was born before July 9, 1322, when Edward II granted him a remainder interest in the manor of Melton Mowbray, which had come into the king's hands thanks to his triumph in the so-called "Despenser war," which had erupted when Hugh the younger's land-grabbing and the king's complaisance outraged their enemies. My educated guess is that Gilbert was born during the unrest, which began with the devastation of the Despensers' lands in May 1321 and ended with the battle of Boroughbridge and the execution of the Earl of Lancaster in March 1322. Had Gilbert been born before the civil war broke out, it seems likely that the king would have given him a present of land at that time or that his father would have made some arrangements to endow him with land, as he seems to have done for the second Despenser son, Edward.
Edward and Gilbert (along with their younger brother, John) were fortunate enough to escape the grimmer consequences of their father's downfall in 1326. At age eighteen, their eldest brother, Hugh, had been entrusted with Caerphilly Castle, which he held against the queen until 1327, when he surrendered it in return for the promise of his life. He was kept a prisoner until 1331, even after the fall of Isabella and Mortimer. Hugh the younger's widow, Eleanor, herself was confined to the Tower, and three of the Despenser girls were forced to take the veil. The boys apparently shared their mother's imprisonment (the order releasing Eleanor in February 1328 refers to her children as well), but once she was free, they were too. One wonders why Isabella and Mortimer, having forced three Despenser girls into convents, were so lax with their brothers. Perhaps having vented their spite on the girls and imprisoned the oldest son, they were content to let the younger boys alone.
Gilbert is next heard from in 1342, when he and his brother Edward fought under the banner of their brother Hugh at Morlaix in Brittany. Sadly, only two of the three brothers came home alive: Edward was killed in the battle.
It is on December 20, 1344, that this mysterious order appears in the Close Rolls:
To Robert de Dalton, constable of the Tower of London, or to him who supplies his place. Order to release Gilbert le Despenser, knight, from prison by the mainprise of William de Bohun, earl of Northampton, and Hugh le Despenser, as he is staying under arrest in the Tower by reason of certain excesses charged against him. By p.s. [16509.]
So what was our friend Gilbert up to? Sadly, I haven't a clue. The maddeningly vague term "certain excesses" appears quite frequently in the Close Rolls, usually in tandem with the equally unenlightening "trespasses." Nothing more is heard of the charges against Gilbert. He had distinguished mainprisers: Hugh, his oldest brother, had worked his way back into royal favor through years of loyal military service, and the Earl of Northampton, a cousin of Edward III and of the Despensers as well, was one of the most distinguished commanders of his day.
Gilbert's offenses, whatever they were (if any), did not hamper his future career. He served at Crecy and Calais, either as one of the king's household knights or under his brother Hugh's banner. In 1349, one John de la Ryvere acknowledged owing 900 pounds to Gilbert. He served Edward III again in 1359-60--his brother Hugh had died in 1349--and with John of Gaunt in 1369. On March 6, 1370, Henry de Wakefield, keeper of the king's wardrobe, gave Gilbert 15/. Os. 7\d. for the wages of himself, his men at arms, and archers. That same year, Edward III granted Gilbert "40 marks yearly, to be received at the Exchequer during his life, for the good service rendered by him to the same Lord the King."
Gilbert died on April 23, 1382. He had been married to Ela de Calverley. The couple had a son, John, who died at age 14 in 1375. Whatever the youthful folly that had put him in the Tower in 1344, Gilbert, aged at least sixty at his death, could congratulate himself on having reached a ripe old age for a Despenser male.