Edward Gagan was 27 years and 8 months old when he enlisted  at Brunswick on 25 May, 1916. A clerk by profession, he lived at 116 Wellington Parade, East Melbourne, with his mother, Janet, and his sisters, Margaret and Helen, a telephonist. In both the 1914 and 1916 Electoral Rolls, Alfred Murray Gagan, probably his brother, also lived at that address. Edward Gagan was sent for training to the 19th Depot Battalion at Geelong, then transferred to the Pioneers training at Seymour. On 12 July, he was attached to the 5th Reinforcements of the 2nd Pioneer Battalion.

The Pioneeer Battalions were not generally engaged in fighting the enemy, but were support troops. Each military Division had a Pioneer Battalion, light military combat engineers who were used to develop and enhance protection and mobility for supported troops through building, construction and maintenance work, road, track and communication preparation. Occasionally they fought as infantry.

The 5th Reinforcements left Australia on 18 September, 1916, sailing from Melbourne on HMAT Karroo A2 on 18 September, 1916, and disembarking at Plymouth on 15 November, where they marched in to join the 2nd Pioneer Battalion, attached to the 2nd Division. From here, they moved to France and the Western Front to join the 2nd Division. The 2nd Division, under the command of Lieutenant General James Legge, had fought at Gallipoli, withdrawnd in december 1916 to Egypt, then went on to serve in france at the Somme. They were initially stationed at Armentieres, a relatively quite sector, but then were in the front line for the Battle of Pozieres, where they sustained 6,846 casualties. They were then engaged at Moquet Farm and Flers. After the severe winter of 1916-17, they were moved in April to the Ypres Sector and the battlesto push the German Army back to the Hindenburg Line. Edward Gagan left for France on 28 February, 1917, on board the SS Golden Eagle, leaving from Folkestone and  was taken on strength with the 2nd Pioneer Battalion on 8 March, 1917.

On 19 June, 1917, Edward Gagan was wounded, not as expected by gunfire, but by an accident to his arm which became septic. He was sent to hospital, where he then suffered an attack of conjunctivitis on 23 June, follwed by another attack on 1 July, which had him posted on to hospital at Rouen. He did not rejoin his battalion until 26 August and on 29 September, 1917was wounded in action, suffering gun shot wounds to his leg and right eye. 

It was four months before he had recovered and was ready to rejoin his battalion. Edward Gagan rejoined the his battalion on 28 January, 1918. He remained until 17 July, when he was gassed and admitted to the 5th Casualty Cleaing Station, then evacuated to Weymouth, England.

On 8 June, 1919, he returned to Australia on board HT Sardinia. he disembarked at Melbourne and was officially discharged from further service on 16 July, 1919. In 1926, the Electoral Rolls record him as a linesman, living at 15 Mayston St., Hawthorn, with his mother and two sisters. By 1931, he had moved to Swan Hill, Victoria, living at Curlewis St and working as a clerk. The last Electoral Roll entry for Edward gagan was in 1949 and  has him living at 188 Knight St., Shepparton and working as a clerk. He may have moved there to be with his brother, Alfred Murray Gagan, of 92 Wyndham St., Shepparton, a process engraver.