Gary Chapman had the following podium finishes at major championships: 3rd in the 1956 Olympics 100 metres Freestyle; 1st in the 1954 British Empire and Commonwealth Games 440 yards Freestyle, 1st in the 1954 British Empire and Commonwealth Games 4×220 yards Freestyle Relay (with David Hawkins, Jon Henricks, and Rex Aubrey), 1st in the 1958 British Empire and Commonwealth Games 4×220 yards Freestyle Relay (with John Konrads, John Devitt, and the non-Olympian J. Wilkinson), 1st in the 1958 British Empire and Commonwealth Games 4×110 yards Medley Relay (with John Monckton, John Devitt, and Terry Gathercole), 2nd in the 1958 British Empire and Commonwealth Games 110 yards Freestyle, 3rd in the 1954 British Empire and Commonwealth Games 1650 yards Freestyle. In the 1958 British Empire and Commonwealth Games 440 yards freestyle Chapman finished fifth.
He died in a boat accident.