John Kingsley Gammage was born at Cootamundra, New South Wales in 1887. At the time of his enlistment in late January 1915, Gammage listed his father, William Gammage, as his next of kin and his occupation as a baker. He departed Australia with the 1st Battalion aboard HMAT Argyllshire on 10 April 1915.

Gammage landed at Gallipoli on 26 June 1915. He was wounded in the left leg during the battle of Lone Pine in August 1915 and was sent to Mudros for treatment and recuperation. He re-joined the 1st Battalion on 25 September 1915. After the evacuation from Gallipoli, Gammage returned to Egypt with the Battalion but was transferred to the Imperial Camel Corps on 29 January 1916. He served with the Corps in Libya and Palestine.

Attached to the Anzac Provost Corps from 16 March 1918, Gammage was transferred permanently when the Imperial Camel Corps was disbanded in June 1918. He received a promotion to extra regimental second corporal on 9 March 1919. John Gammage departed Egypt aboard HMAT Delta to return to Australia on 2 August 1919.