Being under age at his father's death, he was made a ward of Humphrey de Bohun, 2nd Earl of Hereford.
Gilbert de Clare’s castles of Kingston and Tonbridge were taken by the King, Henry III. However, the King allowed de Clare's Countess Alice de Lusignan, who was in the latter, to go free because she was his niece; but on 12 May de Clare and de Montfort were denounced as traitors.
In the following month, by which time they had obtained possession of Gloucester and Bristol, the Earl was proclaimed to be a rebel. However at this point he changed sides as he fell out with de Montfort and the Earl, in order to prevent de Montfort's escape, destroyed ships at the port of Bristol and the bridge over the River Severn at Gloucester.
Having changed sides, de Clare shared the Prince's victory at Kenilworth on 16 July, and in the Battle of Evesham, 4 August, in which de Montfort was slain, he commanded the second division and contributed largely to the victory.
At Michaelmas his disputes with Llewelyn the Last were submitted to arbitration, but without a final settlement. Meanwhile he was building Caerphilly Castle into a fortress. At the end of the year 1268 he refused to obey the King's summons to attend parliament, alleging that, owing to the constant inroads of Llewelyn the Last, his Welsh estates needed his presence for their defence.
At the death of Henry III, 16 November 1272, the Earl took the lead in swearing fealty to Edward I, who was then in Sicily on his return from the Crusade. The next day, with the Archbishop of York, he entered London and proclaimed peace to all, Christians and Jews, and for the first time, secured the acknowledgment of the right of the King's eldest son to succeed to the throne immediately.
Thereafter he was joint Guardian of England, during the King's absence, and on the new King's arrival in England, in August 1274, entertained him at Tonbridge Castle.
Gilbert and Alice separated in 1267; allegedly, Alice's affections lay with her cousin, Prince Edward. Previous to this, Gilbert and Alice had produced two daughters:
After his marriage to Alice de Lusignan was finally annulled in 1285, Gilbert was to be married to Joan of Acre, a daughter of King Edward I of England and his first wife Eleanor of Castile. King Edward sought to bind de Clare, and his assets, more closely to the Crown by this means. By the provisions of the marriage contract, their joint possessions and de Clare's extensive lands could only be inherited by a direct descendant, i.e. close to the Crown, and if the marriage proved childless the lands would pass to any children Joan may have by further marriage.
On 3 July 1290 the Earl gave a great banquet at Clerkenwell to celebrate his marriage of 30 April 1290 with the Joan of Acre (1272 - 23 April 1307). The delay was in getting the Pope to facilitate and agree the arrangement.
Thereafter Gilbert and Joan are said to have taken the Cross and set out for the Holy Land, but in September he signed the Barons' letter to the Pope, and on 2 November surrendered to the King his claim to the advowson of the Bishopric of Llandaff.
Gilbert and Joan had one son - his successor Gilbert, Earl of Gloucestor (1291-1314) who was killed at Bannockburn, and 3 daughters: Eleanor (1292-1337) who married firstly Hugh Despencer (The Younger, favourite of her uncle Edward II)-he was executed in 1326, and she married secondly William de la Zouche; Margaret (1293-1342) who married firstly Piers Gaveston (executed in 1312) and then Hugh Audeley; and the youngest Elizabeth de Clare (16 Sep 1295 -04 Nov 1360), who married John de Burgh, 30th Sept 1308, Waltham Abbey, Essex, England, then Theobald of Verdun in 1316, and finally Roger Damory in 1317. Each marriage was brief, produced one child (a son by the 1st, daughters by the 2nd and 3rd), and left her a widow
In the next year, 1291, he quarrelled with the Earl of Hereford, Humphrey de Bohun, 3rd Earl of Hereford, grandson of his onetime guardian, about the Lordship of Brecknock, where de Bohun accused de Clare of building a castle on his land culminated in a private war between them. Although it was a given right for Marcher Lords to wage private war the King tested this right in this case, first calling them before a court of their Marcher peers, then realising the outcome would be coloured by their likely avoidance of prejudicing one of their greatest rights they were both called before the superior court, the Kings own. At this both were imprisoned by the King, both sentenced to having their lands forfeit for life and de Clare, the Earl of Gloucester, as the aggressor, was fined 10,000 marks, and the Earl of Hereford 1,000 marks.
They were released almost immediately and both of their lands completely restored to them - however they had both been taught a very public lesson and their prestige diminished and the King's authority shown for all.
His extensive lands were enjoyed by his surviving wife Joan of Acre until her death in 1307. Gilbert and Joan had a descendant named Ursula Hildyard of Yorkshire, who in 1596 married (Sir) Richard Jackson of Killingwoldgraves, near Beverley in the East Riding. Jackson died in 1610 and was interred at Bishop Burton. In 1613, James posthumously awarded a coat of arms and a knighthood to Richard for meretorious military service in the Lowlands of Scotland.