Kammala wannan jumla: Gentry ya kammala karatu daga Duntroon a 1919 kuma an tura shi zuwa New Zealand Staff Corps. An tura shi zuwa rundunar sojojin Indiya ta Burtaniya na tsawon shekaru biyu, yana aiki a kan iyakar Arewa maso Yamma yayin yakin Waziristan da kuma
Tawayen Moplah. Bayan ya koma New Zealand, ya rike mukaman ma'aikata da yawa a cikin Yankin Soja na Tsakiya kuma an daukaka shi kyaftin a shekarar 1926. A cikin 1934 an tura shi zuwa Ingila a haɗe da Corps of Royal Engineers na tsawon watanni 18 kuma ya kuma halarci kwasa-kwasan injiniya. Daga nan sai ya yi aiki a Babban Ofishin Janar a Wellington a matsayin jami'in ma'aikata, Injiniyoyi, kafin ya koma Ingila don halartar Kwalejin Ma'aikata a Camberley, inda ya kammala karatu a 1939.