His birth date is confirmed by the Annals of Waverley which record the death “in nocte Sancti Clementis” in 1183 of “Willelmus comes Glocestriæ”, specifying that it was the same day he was born. The Chronica de Fundatoribus et Fundatione of Tewkesbury Abbey records that “comes Willielmus filius et hæres eiusdem” succeeded on the death of Robert Earl of Gloucester. He witnessed an agreement in 1128. He was imprisoned with his father at Rochester in Oct 1141 by the forces of King Stephen.
Governor of Wareham 1144, during his father’s absence in Normandy. He succeeded his father in 1147 as Earl of Gloucester. The Gesta Stephani Regis records that "Willelmus filius suus…senior…ætate" succeeded as Earl of Gloucester on the death of his father, dated from the context to . The Annals of Margan record that “comes Gloucestriæ Willelmus…et comitissa Hawysia” were captured “in castello Cardiviæ” by the Welsh in 1158. “Willielmus comes Glouc.” donated “ecclesia de Eltham” to Rochester, for the souls of “Roberti filii mei...Hawyse comitisse uxoris mee et liberorum...meorum”, by undated charter, confirmed by charter dated 8 Dec 1314.
On his death, King Henry II took the honour of Gloucester and the Earldom into his own hands. Robert of Torigny records the death in 1183 of "Guillermus comes Gloecestriæ" leaving three daughters as his heirs. The Annals of Waverley record the death “in nocte Sancti Clementis” in 1183 of “Willelmus comes Glocestriæ”, specifying that it was the same day he was born. The Chronica de Fundatoribus et Fundatione of Tewkesbury Abbey records the death in 1183 of “comes Willielmus Gloucestriæ” and his burial “apud Kenisham”. The necrology of Lyre monastery records the death "23 Nov" of "Willelmus comes Glocestriæ". 1