Gilbert was born in 1879 to George and Isabella Dennis of Devon, England. He came to Australia at the end of 1913 and was working as a Station Hand at Wallangarra Qld prior to enlisting at Inverell on 10 January 1916. Two days later he joined other volunteers to form the First Contingent of The Kurrajongs who left Inverell for the Narrabri and Armidale camps.
He was a member of the 33rd Battalion, A Company which sailed from Australia on HMAT A74 Marathon on 4 May 1916. On this day, Gilbert was appointed Lance Corporal. After further training at Salisbury Plain, England, his Battalion was sent to the Western Front in France in November. In June 1917 he requested that his rank be reverted to Private. During April 1918 he was gassed and invalided to Beaufort War Hospital in Bristol, England for treatment. After the War, whilst waiting for transport back to Australia, Gilbert worked in mixed farming at Barnstaple for two months. He returned to Australia in September 1919.
The name of Private Gilbert Dennis is inscribed on the Inverell Honor Roll.