Ensign Gillis was assigned to Attack Squadron 45, U.S. Navy, stationed at he Naval Auxiliary Air Station, Cecil Field, Jacksonville, Florida. On 27 November 1950, he was on a routine training mission over the Atlantic Ocean near Mayport, Florida dropping flares for other aircraft to use as targets, when his plane disappeared. He was listed as missing until 1 December 1950, when he was declared dead.

Gillis was awarded the National Defense Service Medal.