Gaselee was born at Brunswick Gardens, Kensington, London, the eldest son of Henry Gaselee, a fellow of King's College, Cambridge, and his wife, Alice Esther. His great grandfather was Sir Stephen Gaselee, a justice of the Court of Common Pleas. He attended Temple Grove School in East Sheen. In 1896, he entered Eton College, where he was elected a King's scholar and edited the Eton College Chronicle. In 1901, he entered King's College, Cambridge University. In 1904, he earned a first class in part 1 of the classical tripos and, in 1905, a second class in part 2. In 1905, he left university to become tutor to Prince Leopold of Battenberg, a grandson of Queen Victoria, and travelled widely. In 1907, he resumed his studies at Cambridge, where he was an editor of the Cambridge Review. In 1908, he became the Pepys Librarian at Magdalene College, Cambridge. In 1909, he was elected a fellow, which he held for 34 years. From 1916 to 1919, he worked with the Foreign Office. He returned to Cambridge after the war. But, in 1920, he was made librarian and Keeper of the Papers at the Foreign Office, a post he held till his death. He was president of the Bibliographical Society of London from 1932 to 1934. Gaselee's recreations, according to the anonymous obituary in The Times, were "travel, shooting and bridge," but he was a man of wide interests for whom work and recreation blended imperceptibly. He wrote on "classical literature, medieval and modern Latin... Coptic, hagiography and liturgiology, palaeography and bibliography, Spain, Portugal, Madeira, wine and food". His friend Ronald Storrs characterised him during their undergraduate days as follows:
Stephen Gaselee was already at the age of twenty what he never ceased to be, a Cambridge Personality; Gaselee, with almost as many friends as interests, a first-class classical scholar, a bibliophile, a bibliographer, a liturgiologist; Gaselee, who when playing tennis wore his hair in a net; who kept Siamese cats, fed with a revolting portion of cow’s lung preserved on a plate above his bookshelf; who had a fire every day in the year because England has a cold climate; who founded the Deipnosophists’ dining club, where the members, robed in purple dinner-jackets lined with lilac silk and preluding dashingly on Vodka, would launch forth into an uncharted ocean of good food and even better talk; Gaselee, who read, wrote and spoke Ancient Coptic ; Gaselee, nightly puffing his long churchwarden whilst he expatiated on Petronius, vestments, Shark’s Fin and cooking problems; a lay Prince of the Church, Ecclesiastic Militant and Gastronomer Royal.
He was a frequent and generous donor of books to Cambridge University Library. One unusual item was acquired at Sinaia in 1926: a copy, signed to Gaselee, of Queen Marie of Romania's novel Why? A story of great longing. His major donations were a collection of 311 incunabula, given in 1934; 279 early 16th century books, given in 1940; and fifty books to be chosen by the Librarian at his death. A group of benefactors bought his personal collection of works relating to Petronius and the Satyricon from his heirs and gave it to Cambridge University Library.
