Gilbert came from a long line of de Clares: Richard fitz (i.e. son of) Gilbert de Clare came to England in 1066 with William the Conqueror, and his great-grandson Gilbert de Clare was created first earl of Hertford by King Stephen in about 1138. This Gilbert's great-grandson, also Gilbert, born in 1180, married his third cousin Isabella Marshal, one of the five daughters of William Marshal, earl of Pembroke and his wife Isabella de Clare (from another branch of the de Clare family, who had become earls of Pembroke also by creation of King Stephen in 1138). Gilbert de Clare and Isabella Marshal were the parents of Richard, earl of Gloucester and Hertford (1222-1262), himself the father of Gilbert 'the Red' born in 1243 and died in 1295, who married Edward I's daughter Joan of Acre in 1290 and was the father of Gilbert born in 1291. The Gilbert de Clare who lived from 1180 to 1230 and married Isabella Marshall inherited the earldom of Gloucester from his mother Amicia and her sister Isabel (or Hawise or Avisa) of Gloucester, the latter the first wife of John, king of England, granddaughters and heiresses of Henry I's eldest illegitimate son Robert, earl of Gloucester.
Dower was assigned to Gilbert de Clare's widow Maud de Burgh, the customary one-third of his lands and with a value of £2222 a year, on 5 December 1314.  As Maud continued to claim to be pregnant until at least February 1316, the division of Gilbert's lands among his three sisters Eleanor, Margaret and Elizabeth, his co-heiresses, was delayed, and was delayed still further when some of the juries taking part in the Inquisitions Post Mortem for him held in numerous counties wrongly declared that one of his heirs was named Isabel rather than Elizabeth. Isabel de Clare was Gilbert's much older half-sister and not his heir. Edward II finally ordered the partition of the de Clare inheritance among the three sisters in April 1317, and they and their husbands took possession of them that November. The schedules dividing up the lands still exist in the National Archives in Kew (C 47/9/23, C47/9/24, C 47/9/25).
The eldest sister Eleanor de Clare and her husband Hugh Despenser the Younger had lands in England, Wales and Ireland to the value of £1497 plus a reversion of £946 on the death of Countess Maud in 1320, to a total of £2443. Eleanor and Hugh's most important possession was the rich lordship of Glamorgan in South Wales. The second sister Margaret de Clare and her husband Hugh Audley had lands in England, Wales and Ireland to the value of £1384 plus a reversion of £928 in 1320, making a total of £2314. The third sister Elizabeth de Clare and her husband Roger Damory had lands in England, Wales and Ireland to the value of £1391 plus a reversion of £881 in 1320, to a total of £2274. This wealth immediately catapulted all three men to the forefront of the nobility.
Eleanor's share passed on her death in June 1337 to her eldest son Sir Hugh Despenser, who was then in his late twenties. Hugh died childless in 1349, and the lands passed to his nephew Edward, born in 1336, eldest son of Eleanor de Clare's second son Edward, who was killed at the battle of Morlaix in 1342. On the death in 1375 of the younger Edward - the famous Kneeling Knight of Tewkesbury Abbey - the Despenser lands passed to his son Thomas, born in 1373, who was briefly earl of Gloucester in the late 1390s and summarily beheaded in January 1400 after taking part in the Epiphany Rising to restore the deposed Richard II to the throne. Thomas's sons died young and his heir was his posthumous daughter Isabel, born on 26 July 1400 six and a half months after Thomas's death, who married two men called Richard Beauchamp. The second of these was the powerful earl of Warwick (1382-1439), guardian of the young Henry VI in the 1420s. Isabel Despenser and Richard Beauchamp's ultimate heir (after the death of their son Henry and his young daughter) was their daughter Anne Beauchamp, born in 1426, who took the Despenser lands and the earldom of Warwick with her on her marriage to Richard Neville, the 'Kingmaker'. I assume the Despenser lands and thus the third of the de Clare inheritance of 1314 then passed to either George of Clarence or his brother Richard of Gloucester, Anne Beauchamp and Richard Neville's sons-in-law, but I don't know which one.
Margaret's share passed in 1342 to her daughter Margaret Audley, born c. 1320, her only surviving child after the death of Joan Gaveston in January 1325. Margaret Audley was abducted and forcibly married to Ralph Stafford, later first earl of Stafford, in 1336, and her inheritance of the third of the de Clare lands passed to her eldest surviving son Hugh, second earl of Stafford, born in about 1341, and then through a long line of Staffords in the fourteenth and fifteenth centuries. Margaret Audley's great-grandson Humphrey Stafford, born at the beginning of the 1400s, was created first duke of Buckingham; other descendants who inherited the de Clare lands included Duke Henry, executed by Richard III in 1483, and his son Duke Edward, executed by Henry VIII in 1521.
Elizabeth's share: Elizabeth's only son William Donn de Burgh, earl of Ulster, was killed in June 1333 when he wasn't even twenty-one years old, leaving a baby daughter, Elizabeth de Burgh, born in July 1332, who inherited the earldom of Ulster and her paternal grandmother's third of the de Clare lands in 1360. The great heiress was married to Edward III's second son Lionel, duke of Clarence, and her inheritance passed to her only daughter Philippa, born in 1355, who married Edmund Mortimer, earl of March (great-grandson of Roger Mortimer executed in 1330). Thus Elizabeth de Clare's inheritance came to the Mortimer family and stayed with them until the death of Edmund Mortimer, grandson of Philippa of Clarence and Edmund Mortimer, in 1425, when it passed to his sister Anne's son Richard, duke of York (1411-1460), who was also the heir of his paternal uncle Edward, duke of York (killed at Agincourt in 1415). Duke Richard was the father of Edward IV and Richard III.
1) Calendar of Documents Relating to Scotland 1307-1357, p. 41.
2) Calendar of Close Rolls 1313-1318, pp. 131-139; T.B. Pugh, 'The Marcher Lords of Glamorgan and Morgannwg, 1317-1485', Glamorgan County History, III: The Middle Ages, ed. T.B. Pugh (1971), p. 167.