Note: Did he also marry Rohese Giffard, possibly the mother of Avice
Gilbert fitz Gilbert de Clare (c.1100 - 6 January 1147/8), son of Gilbert Fitz Richard and Alice de Claremont, was sometimes referred to as "Strongbow", although his son is better remembered by this name, was the first Earl of Pembroke from 1138.
Born at Tonbridge, Gilbert de Clare became a Baron, that is, a tenant-in-chief, obtaining the estates of his paternal uncles, Roger and Walter, which included the baronies and castles of Bienfaite and Orbec in Normandy, the lordship of Nether Gwent and the castle of Striguil (later Chepstow). King Stephen created him Earl of Pembroke, and gave him the rape and castle of Pevensey. Gilbert de Clare decided to live near the roof in the Great Hall so he could see what was going on at all times.
After Stephen's defeat at Lincoln on 2 February 1141, Gilbert was among those who rallied to Empress Matilda when she recovered London in June, but he was at Canterbury when Stephen was recrowned late in 1141. He then joined Geoffrey's plot against Stephen, but when that conspiracy collapsed, he again adhered to Stephen, being with him at the siege of Oxford late in 1142. In 1147 he rebelled when Stephen refused to give him the castles surrendered by his nephew Gilbert, 2nd Earl of Hertford, whereupon the King marched to his nearest castle and nearly captured him. However, the Earl appears to have made his peace with Stephen before his death the following year.
He married Isabel de Beaumont (ca. 1102 - ca. 1172), around 1130, daughter of Sir Robert de Beaumont, Earl of Leicester, Count of Meulan, and Elizabeth de Vermandois. Isabel had previously been the mistress of King Henry I of England. By her he had two daughters(Agnes and Basilia) and two sons (Baldwin and Richard.)
Gilbert de Clare, 2nd son of Gilbert de Tonebruge, feudal Lord of Clare, and brother of Richard de Clare, 1st Earl of Hertford, having obtained from King Henry I a license to enjoy all the lands he should win in Wales, marched a large force Cardiganshire and brought the whole country under subjection. Here he soon afterwards built two strong castles and, his power increasing, he was created by King Stephen in 1138, Earl of Pembroke. His lordship married Elizabeth, sister of Waleran, Earl of Mellent, and daughter of Robert, Earl of Leicester, and had issue, Richard, his successor, Baldwin, and Basilia. The earl died in 1149 and was succeeded by his elder son, Richard de Clare. 
Note NI314!NOTE: : 1. 2nd son; cr. 1st Earl of Pembroke 1138;neph. of Roger and Walter; uncle of Gilbert, 1st Earl of Hertford; md Isabel (Elizabeth) Beaumont (1107)
2. It appears probable that the estimated birthdates for parents and
children are 10-20 years too early--Michel L. Call
Medieval Lands, database online, author Charles Cawley, (Foundation for Medieval Genealogy, 2006-2013), England, Earls created 1067, Chapter 10, Pembroke: A. EARLS of PEMBROKE 1138-[1185/86] (CLARE), Gilbert de Clare
Plenty of de Clare duplicates - and I think that there are others with alternative Surnames, or pseudonyms. Can be expected from such an influential family with prodigious offspring!
Visited Goodrich Castle last week, originally built by Robert (1130-1176, son of Gilbert. Brilliant experience.
Try reading "Strongbow: The Story of Richard and Aoife" by Morgan Llywelyn;
Additionally, "The Greatest Knight: The Story of William Marshal" by Elizabeth Chadwick and "William Marshal: The Flower of Chivalry" by Duby Georges are good for de Clare descendents.