[2] Promoted to lieutenant on 15 February 1892, he saw active service in West Africa 1893–94 in operations against the Sofas,[3] and in 1897 joined the geographical section of the Intelligence Branch of the War Office.

In July 1915, he was sent to the Middle East and was appointed General staff Officer Grade 1 (GSO1) of the Australian 2nd Division at Gallipoli.

He was eventually posted to serve as the chief of staff of the II Anzac Corps, a position he held until the end of the war.

During the Great War he was mentioned in dispatches six times, received the brevet ranks of lieutenant-colonel and colonel, and was awarded the Belgian Croix de Guerre and the French Légion d'honneur.

[6] After his retirement, in 1934, Major General Gwynn wrote Imperial Policing, now regarded as a classic in the field of low intensity conflict and small wars.