He died unmarried. Whether he was ever made Earl (he might have been the first in the family if so, because it is not certain if his father was.) Complete Peerage says the following:
GILBERT DE CLARE, Lord of Clare, &c., s. and h., b. before 1115, was at some date before 1142 (not improbably in 1138, when his uncle and namesake is supposed to have been cr. Earl of Pembroke)(a) cr. EARL OF HERTFORD.(b ) He d. unm. 1152, and was bur. at Clare Priory.
For the two footnotes we can trace these remarks:
(a) [Complete Peerage refers to another footnote which concerns the fact that Gilbert's father's uncle named Gilbert was made Earl of Pembroke. Although he was created Earl by King Stephen, his son Richard was also Earl of Pembroke.]
(b) In Courthope, under " Pembroke," is the following note: " King Stephen, 1138, says William of Malmesbury,' Multos Comites qui ante non fuerant, instituit, applicatis possessionibus et redditibus quse proprio jure Regi competebant.' They were afterwards called imaginary and false Earls, and Henry II, at the beginning of his reign, ' deposuit quosdam imaginarios et Pseudo Comites (says the author of a Chronicle of Normandy) quibus Rex Stephanus omnia pene ad fiscum pertinentia minus caute distribuerat.'" See some account of the Earldoms conferred by King Stephen in vol. iv, Appendix D.
Complete Peerage, 2nd ed, Vol.3, p.244
"Royal Ancestry" 2013, Douglas Richardson Vol. II. p. 174
WikiTree profile Hertford-6 created through the import of Spencer Family Tree 4 2002.GED on Nov 28, 2011 by Chet Spencer. See the Changes page for the details of edits by Chet and others.