The first Earl of Pembroke, created 1138. (Browning, Charles. Americans of royal descent. 7th ed. Baltimore : Genealogical Pub. Co., 1986, pg. 201). aka Gilbert Fitz-Gilbert de Clare (Browning, pg. 197). aka Gilbert Fitz-Gilbert de Clare (Browning, pg. 339).
Gilbert Fitz Gilbert or de Clare was made Earl of Pembroke in 1138 by King Stephen, his elder brother Richard Fitz Gilbert or de Clare being father of the Alice/Adelaide who married William de Percy. In the period of unrest and indecisive civil war known as the Anarchy, when followers of Stephen, Henry I's nephew, struggled with the Empress Maud, Henry I's daughter, Gilbert Earl of Pembroke sided with each one. He already held Chepstow, on the Welsh-English borders. In 1144 he pushed far into South Wales and established himself at Carmarthen.
He married Isabel de Beaumont, before 1130, daughter of Sir Robert de Beaumont, Earl of Leicester, Count of Meulan, and Elizabeth de Vermandois (findagrave.com)