A swimmer from a young age, Gary MacDonald’s first major international tournament was the 1973 Summer Universiade, where he represented Canada as a student at Simon Fraser University. His next stop was the 1974 Commonwealth Games, where he took gold in the 4x100 metre freestyle relay (alongside Ian MacKenzie, Brian Phillips, Bruce Robertson) and bronze in the 100 metre butterfly and the 4x200 metre freestyle relay. He picked up two silver medals in relay events at the 1975 Pan American Games and was then selected to compete at the 1976 Summer Olympics, where he earned silver in the 4x100 m medley relay (with Robertson, Clay Evans, Steve Pickell, and Graham Smith) and was eliminated in the semifinals of the 100 metre freestyle event.
After the Olympics MacDonald entered York University for graduate studies, but continued to compete for Canada through 1978, when he participated in the World Championships and the Commonwealth Games. At the latter he captured gold in the 4x100 m freestyle relay (alongside Smith, Bill Sawchuk, and Peter Szmidt) and bronze in the 100 m freestyle. Following his retirement from active competition, he took up coaching and spent two decades as the head coach of the Halifax Aquatic Club. As of 2013 he works as an assistant coach with the Dalhousie University Tigers in Halifax, Nova Scotia. During his career he held national records in the 50 and 100 metre freestyle events, as well as the 200 metre individual medley.