Allen was born into a clerical family, being the eldest son of The Reverend T.K. Allen, sometime Vicar of Weyhill. He was educated at Cheltenham College, later serving as a member of the College Council (1923–51) and President of the College (1939–51). He was a scholar of Wadham College, Oxford, earning first-class honours in the Final Honour School of Theology in 1908, and in 1910 being elected Denyer and Johnson Theological Scholar and receiving the Ellerton Essay Prize. He studied briefly at Wells Theological College in 1908 and was ordained deacon the same year, when he had just satisfied the canonical requirement for candidates for ordination to have attained twenty-three years of age. Both his youth and the brevity of his training were quite normal at the time.
His first appointment was as Chaplain to Wadham (1908–10) and he was ordained priest in 1909. From 1910 until 1920 he was Fellow, Dean, and Chaplain of Pembroke College (Honorary Fellow 1934). He was Temporary Chaplain to the Forces 1917-18 and Chaplain to the Royal Air Force 1918-19. In 1920 he returned to Oxford as Senior Proctor (1920–21) and Principal of St Edmund Hall (1920–28; Honorary Fellow 1942), assuming office at the age of just thirty-five. From 1923 until 1928 he was a member of the Hebdomadal Council of the University of Oxford.
From 1928 until 1936 he was the second Bishop of Sherborne. He returned to Oxford in 1936, where he was Archdeacon of Oxford and Canon of Christ Church until 1952. He was also Assistant Bishop of Oxford 1936-39 and served as the first modern Bishop of Dorchester 1939-52.
- Who was Who
- The Times (1910)
- The Times (1951)
- The Times (1936)
- The Times (1956)
- The Times (1910) "University Intelligence: The University Endowment Fund Thursday", 19 May (39276), p. 3 (col. 'C')
- The Times (1936) "Ecclesiastical News", 21 August (47459), p. 13 (col. 'G')
- The Times (1951) "St Edmund Hall, Oxford: Institution of New Principal", 13 October (52131), p. 8 (col. 'F')
- The Times (1956) "Dr. G. B. Allen Former Bishop Of Dorchester", 29 March (53492), p. 14 (col. 'D')
- Who was Who (1991) Who was who. A cumulated index 1897-1990, CD-ROM, London : A & C Black, ISBN 0-7136-3457-X
|Church of England titles|
Robert Crowther Abbott
| Bishop of Sherborne
1928 – 1936
John Maurice Key
| Bishop of Dorchester
1939 – 1952