Gregg was the son of a mechanical engineer who was involved in the manufacture of marine incinerators.
He began competing in gymkhanas and ice races after an initial appearance in a hill climb in 1958 in Laconia, NH. After his graduation from Harvard, he moved to Europe and attended the Centro-Sud Driving School. He than enlisted in the U.S. Navy and became an Air Intelligence Officer, winding up in Jacksonville, FL.
In 1962, he pursued his motorsports career with the SCCA. In April of 1963, in Osceola in a completely stock production Corvette, he won. By the time he was discharged from the Navy in 1965, racing and Jacksonville were in his blood.
He took over a local Porsche agency and by 1968, it had prospered enough for him to acquire a Mercedes Benz dealership. By 1970, he had a third dealership, this one called SportAuto, which offered Fiats and MGs. He became a serious Porsche racer in 1964 with a 904 and then moved into competition with a Carrera. He was the SCCA's Southeastern Division champion in 1967 in two classes and had scored victories in Daytona and Sebring.
In 1968, he entered competition in the SCCA's Under-2-Litre section of the Trans-Am Series. He won six Trans-Ams and the title in 1969 and also took the SCCA's B Sedan National Championship.
In 1971, he was part of the major Trans-Am Series, driving Bud Moore Ford Mustangs. He won the Trans-Am Championship in 1973 and again in 1974. By this time, he was involved with IMSA and scored his greatest achievements with the IMSA GTO overall championship in 1971 and 1973. It was also in 1973 that he had his greatest moment as a road racer when he won the 24 Hours of Daytona in a Porsche Carrera co-driven by Hurley Haywood. He then impetuously announced his retirement, preferring to lead a life as a director of the Jacksonville National Bank, a club tennis star and a speedboat racer out of the exclusive Ponte Verde Yacht Club.
But he retracted his retirement and went on to win the 24 Hours of Daytona two more times, in 1975 and 1979. In IMSA, he was the dominant force with, astonishingly, GTO overall championships in 1974, 1975, 1978, and 1979, giving him six career titles in the class.
At le Mans in 1980 he sustained head injuries enroute to the circuit on the morning of the race, coliding with a horse and cart. Whether it was a result of this or the suggestion that he was suffering from a progressive and incurable nervous system disorder, never the less, on December 15, 1980 at age 40 and shortly after getting married to Deborah, he went out into the sand dunes near his home and shot himself. The official finding was suicide.
At the time of his death in 1980, Gregg had achieved a reputation as one of America's greatest and most successful road racers.