Gunton was born in Santa Monica, California, the son of Rose Marie (née Banovetz) and Robert Patrick Gunton, Sr., a labor union executive. He attended Mater Dei High School in Santa Ana, California, the Paulist Seminary St Peter's College, in Baltimore, Maryland, and the University of California, Irvine. A lifelong devout Roman Catholic, Gunton had initially planned to become a priest. Gunton served in the United States Army (1969–71), earning a Bronze Star for valor and the Vietnam Service Medal.