Gerard Henderson Cowan was born on 19 November 1895 at Geraldton, Western Australia to Robert Henderson Cowan and Rosa Howard (née du Boulay) Cowan. He was the middle child of three living siblings, having an elder sister and a younger brother. Cowan worked on the family farm at Narngulu, Western Australia, and studied law.
Cowan enlisted to the Australian Imperial Force on 3 July 1916 as a private, and was assigned to the 11 Australian Infantry Battalion, 22nd Reinforcements. On 9 November 1916 he embarked for overseas service on the HMAS Argyllshire from Fremantle, Western Australia, and landed in England on 10 January 1917.
After a period of training, Cowan proceeded to France to serve with his battalion. On 21 September 1917 he was wounded in action, receiving a gunshot wound while carrying a wounded fellow soldier to a dressing station. He was sent to hospital in England to recover. After his convalescence, Cowan was accepted into the 1st Infantry Brigade Signal School and was subsequently accepted into the Australian Flying Corps (A.F.C.) on 1 March 1918. He trained with 6 Training Squadron and qualified as a pilot on 1 October 1918. He then served with the 2nd Squadron, A.F.C., in France. Cowan was promoted to Lieutenant on 1 January 1919.
While serving in France, Cowan was admitted to hospital on 15 February 1919. He suffered from influenza, which developed into broncho-pneumonia. Gerard Henderson Cowan died on 22 February 1919, at the age of 23. He is buried at Longuenesse Souvenir Cemetery, near Saint Omer, France.