Gerard was the son and heir of Sir Warin de Lisle and his wife, Alice, sister and heiress of Henry Teyes Junior, Lord Teyes. In March 1327, when proceedings against his executed father, as an adherent of Thomas, Earl of Lancaster, were annulled, the escheator was ordered not to meddle with his lands, as the Sir Warin had held nothing in chief. Gerard was knighted in the same year and was summoned by the King for military service in December 1334 and March 1335. In 1333 and 1335, he was certainly engaged in the Scottish Wars of King Edward III, in the latter year under Richard, Earl of Arundel. In 1339, Sir Gerard had a dispute with his mother as to the presentation to the church of Stowe-Nine-Churches (Northamptonshire), but he eventually admitted that it was not his turn. He also complained of trespass in Stowe Park, which appears to have been his preferred residence, though he must also have spent a considerable amount of time at Kingston Castle in Berkshire, when attending the King in Woodstock, Oxford, Windsor or London.

In June 1340, Sir Gerard was again in the army of the Earl of Arundel, this time travelling ‘beyond the seas’ as a knight banneret, engaged in the French campaign of 1346-47. He fought at the Battle of Crecy in the Earl's division. In 1347, Sir Gerard succeeded to his mother's inheritance and, according to modern doctrine, became Lord Teyes, although it seems unlikely that he ever used the title. He made an agreement, on 5th March 1350, to again accompany the Earl of Arundel, with 30 men-at-arms in return for 100 marks (£66-13s-4d) per annum, receiving £100 before leaving England.

In September 1350, Sir Gerard received a licence to make a pilgrimage to Rome, along with seven horses. This he presumably undertook shortly afterwards, perhaps in thanks for his safe return from the Hundred Years War. In August 1354, as Gerard de Insula, dominus de Stowe, he was one of the barons who agreed to the appointment of the Pope as arbitrator in the continuing dispute with France, and, on 20th June 1358, he attended the King at a Royal Council. Eventually, on 15th December 1357, Sir Gerard was summoned to Parliament by writs directed to ‘Gerardo de Insula or del Isle’, whereby he is held to have become Lord Lisle (of Kingston Lisle). In Sep 1359 he was again engaged in the French wars.