Gallagher studied English at the University of Pennsylvania for three years after graduating from Devon Preparatory School in 1964. In June 1966, he enlisted in the Marine Corps, and was a wireman and radio telephone operator with the Headquarters and Service Company of Battalion Landing Team 3, 1st Marines, 9th Marine Amphibious Brigade, in Vietnam. The lance corporal, 23, died in Quang Tri Province during Operation Lancaster II on March 8, 1968. He was survived by his parents, a brother and a sister.