Gabriel was born in Lincolnton, North Carolina, on January 21, 1928. Following graduation from high school, he attended Catawba College, Salisbury, North Carolina, for two years before entering the United States Military Academy. He graduated from the academy in 1950 with a bachelor of science degree and a commission in the U.S. Air Force. The general earned a master of science degree in engineering management from The George Washington University, Washington, D.C., in 1963. He graduated from the Command and Staff Course at the Naval War College, Newport, Rhode Island, in 1962; and the Industrial College of the Armed Forces, Fort Lesley J. McNair, Washington, D.C., in 1967.