Born in Kansas City, Gregory was the heir to an insurance company fortune. He was fast but accident-prone but he began to make his mark on the international scene in 1957 when he shared a Ferrari with Cesare Perdisa, Eugenio Castellotti and Luigi Musso to win the World Sportscar Championship event in Buenos Aires. Later that year he made his first appearance in F1, driving a Scuderia Centro Sud Maserati. He finished third in Monaco and fourth at Pescara. His best result came in Portugal in 1959 at the wheel of a factory Cooper-Climax. In total he competed in 38 Grands Prix. In addition he was a regular competitor in international sportscar events, winning the Nurburgring 1000 event with Lucky Casner in a Maserati in 1961. Four years later he raced at Indianapolis for the first time. He started from the back of the grid but worked his way up to fifth place before being sidelined with an engine problem. A few weeks later he scored his most famous victory at the Le Mans 24 Hours, sharing a NART Ferrari with Jochen Rindt. Gregory then began to wind down his motor racing career, continuing to compete in international sports car races with some good results including a second-place finish at the 1966 1000 km race at Monza alongside John Whitmore. Following his good friend Jo Bonnier's death at the 1972 Le Mans race, Gregory stopped racing, and retired to Amsterdam, where he worked as a diamond merchant before operating a glassware business. On November 8, 1985, Gregory died in his sleep of a heart attack at his winter home in Porto Ercole, Italy. He had three children, Debbie, Scott and Michael.