[2] Educated at Dulwich College, Gillman was commissioned into the Royal Field Artillery in July 1889.

He served in the Second Boer War 1899–1900, where he took part in the Relief of Kimberley, and the following battles of Paardeberg (late February 1900), Poplar Grove, and Driefontein (March 1900).

[3] In April 1908 he served as an assistant military secretary to Lieutenant General Sir Arthur Paget, who had recently been placed in charge of Eastern Command.

[8] He served in the First World War, being seconded in January 1915 to the 13th (Western) Division as its general staff officer, grade 1 (GSO1).

[3] He was then promoted to temporary major general in June 1916[12] while serving with the British Salonika Army from 1916 to 1917 before becoming commander of the 17th Indian Division in August 1917.