First Lieutenant Gallagher was the pilot of a B-26B Invader bomber with the 8th Bomber Squadron, 3rd Bomber Group. On 30 September 1950, while on a night intruder mission, the aircraft was hit by anti-aircraft fire, crashed into the ocean near Wonsan, North Korea, and exploded. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Gallagher was awarded the Air Medal with Oak Leaf Cluster, the Purple Heart, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.