When the First World War broke out in 1914 Gossage was still a young second lieutenant in the Royal Field Artillery, where he remained for the first year of the war. However, on 12 May 1915, he was seconded to the Royal Flying Corps where he was assigned to No. 6 Squadron as a pilot. By 5 September 1915 he had reached the rank of captain and had become a Flight Commander in No. 6 Squadron. He was awarded the Military Cross in March the following year for his "consistent good and zealous work under bad weather conditions, both on patrol and when co-operating with the artillery in operations resulting in the capture of the enemy's position".