Gunn began his automotive career as a salesman at Smith Chevrolet, San Antonio, Texas, in 1934. In 1935, he was named Sales Manager and three years later was made General Manager. In 1945, upon his return from military service in World War II, he returned to his duties as General Manager and was named a Partner of Smith Chevrolet. In 1955, Gunn sold his interest in Smith Chevrolet and established Gunn Oldsmobile. Gunn successfully built one of the first megadealership operations with the opening of Mission Chevrolet in 1967, (now Gunn Chevrolet-GEO), Gunn Honda in 1976, Gunn Nissan in 1977, Gunn Buick and Gunn Acura in 1988, Gunn Infiniti in 1991, and Gunn Dodge in 1992. Gunn has received many awards over the years including the Texas Quality Dealer Award in 1965, the Benjamin Franklin Quality Dealer Award in 1966, and the TIME Magazine Quality Dealer Award — National Finalist in 1980.