Brother of 1566 Trooper George Gavin, 482 Private James Gavin (KIA), 3261 Private Patrick Joseph Gavin and 3262 Private Stephen Gavin.
John Gavin transferred to the Australian Artillery Reserves in England in August 1917. In September he married Edith Ethel Plank. He arrived in France in January 1918 allotted to the 1st Field Artillery Brigade. In October he contracted influenza and, when he developed pneumonia, was evacuated to Birmingham War Hospital. He returned to Australia in January 1919 and was discharged in April.