Gregory participated in 43 Formula One Grands Prix, predominantly with privateer teams; he also competed in numerous non-championship races, winning the 1962 Kanonloppet with BRP.

Gregory was also successful in sportscar racing, entering 16 editions of the 24 Hours of Le Mans between 1955 and 1972, winning in 1965 alongside Jochen Rindt, driving the Ferrari 250LM.

At the end of 1953, Gregory was invited to his first international sports car race - the 1954 1000 km Buenos Aires in Argentina, which he finished in 14th due to water pump problems.

[2] 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).

Gregory scored a pole position and set a course record at the non-Championship race at Aintree, but his contract with Cooper was not renewed for the following year.

Gregory's early years of competition were marked by many crashes, often the result of pushing sub-par machinery past its ability.

He flipped a thankfully rollbar-equipped Maserati at the Venezuelan Grand Prix in 1957, totalled two sports cars in 1958, and another two in 1959 (a Lister-Jaguar and a Tojeiro-Jaguar).

He was unable to reproduce the results he obtained early in his career, his best being a sixth at the 1962 United States Grand Prix at Watkins Glen with the UDT Laystall team, in a Lotus 24.

After his release from Cooper, Gregory also went back to competing in sports car races, setting the overall fastest lap at the 1960 24 Hours of Le Mans.

He won the 1961 1000 km Nürburgring, driving alongside Lloyd "Lucky" Casner in a Maserati Tipo 61 for the America Camoradi Racing Team.

1965 was also the year in which Gregory raced in the Indianapolis 500, starting from the back of the grid and working his way up to 5th before being forced to retire due to an engine problem.