Gimson assumed the post of the Colonial Secretary of Hong Kong in December 1941. However, his appointment was interrupted by the Battle of Hong Kong. He became a prisoner of war when then Governor Sir Mark Young surrendered to the Imperial Japanese Army on Christmas Day 1941. After spending more than three years in Stanley Internment Camp as an internee, Gimson was freed in August 1945, upon the Liberation of Hong Kong. He formed a short-lived provisional government and briefly declared himself "acting governor", but this administration was soon replaced when Rear Admiral Cecil Harcourt established the military government following in September.