Gentry graduated from Duntroon in 1919 and was posted to the New Zealand Staff Corps. He was seconded to the British Indian Army for two years, serving on the North-West Frontier during the Waziristan campaign and the Moplah rebellion. On returning to New Zealand, he held a number of staff positions in the Central Military District and was promoted to captain in 1926. In 1934 he was sent to England on attachment to the Corps of Royal Engineers for 18 months and also attended engineering courses. He then served at General Headquarters in Wellington as staff officer, Engineers, before returning to England to attend the Staff College at Camberley, graduating in 1939.