Throughout 1954 and 1955, Gregory competed in European races, usually driving Ferraris. His record includes the Tourist Trophy at Dundrod and the 24 Hours of Le Mans (although his co-driver Mike Sparken retired before Gregory got a chance to drive). He also won the inaugural Nassau Trophy at the Bahamas Speed Week in 1954. Moving back to America in 1956, Gregory entered several SCCA races, often winning. In 1957, he had another attempt at the Argentine 1000 km race, this time winning. This performance got him a drive with Guglielmo Dei's Scuderia Centro Sud, a privateer Formula One team using the Maserati 250F. His first race was the 1957 Monaco Grand Prix, where he scored an impressive third-place finish, the first podium for an American in an F1 Grand Prix. He followed this with a string of good results, coming eighth in the German Grand Prix, and fourth in both the Pescara and Italian Grands Prix. Despite only competing in half of the races, Gregory ended the 1957 season in sixth place in the championship.