His father, however, was killed in World War I and in 1920 Gentzen's mother moved to Stralsund. Gentzen had already begun his secondary schooling at this stage but he continued his education at the Humanistische Gymnasium in Stralsund.
Kanyang ama, gayunman, ay namatay sa World War I at sa Gentzen's 1920 ina inilipat sa Stralsund. Gentzen nagkaroon na sinimulan ng kanyang pang-ikalawang-aaral sa hakbang na ito ngunit ang patuloy na niya ang kanyang pang-edukasyon at ang Humanistische himnasyo sa Stralsund.