Jacob was born in Upper Norwood, London, the seventh son and youngest of ten children of Stephen Jacob, and his wife, Clara Laura, "née" Forlong. Stephen Jacob, an official of the Indian Civil Service based in Calcutta, died when Gordon was three. Jacob was educated at Dulwich College, and enlisted in the Queen's Royal Regiment (West Surrey) at the outbreak of the First World War. He was taken prisoner in 1917, and in the prison camp he studied a harmony textbook in the camp library and began composing. He wrote for an orchestra of his fellow prisoners, with assorted instruments. After the war he studied journalism before turning to music. He took a correspondence course, gained an ARCM diploma and was accepted as a full-time student at the Royal College of Music (RCM) in 1920. There, he was a pupil of Charles Villiers Stanford and Ralph Vaughan Williams (composition), Herbert Howells (music theory) and Adrian Boult (conducting), from whom he learned the "economy and decision" of his podium technique.