From 1981 through 1986, Marcel Gery represented his native Czechoslovakia on the international stage, beginning with seventh place finishes in the 100 and 200 metres butterfly at the European Championships. His only major podium finish with this country was third at the 1985 European Championships in the 100 m butterfly. Following fourth, fifth, and sixth place finishes in the 200 m butterfly, the 200 m freestyle, and the 100 m butterfly respectively at the 1986 World Championships, Gery moved to Canada and began competing for that country. His first major success came at the 1987 Pan Pacific Championships, where he was third in the 200 m freestyle. He qualified for the 1988 Summer Olympics, but could not attend as he had not yet acquired Canadian citizenship. Two years later, at the 1990 Commonwealth Games, he earned three medals: silver in the 100 m butterfly, bronze in the 4x100 m freestyle relay (alongside Blair Hicken, Stephen Vandermeulen, Darren Ward, and the non-Olympian Stephane Herbert), and gold in the 4x100 m medley relay (with Jon Cleveland, Tom Ponting, and Mark Tewksbury). His next stop was the 1991 Pan Pacific Championships, where he took silver in the 100 m butterfly and the 4x100 m medley relay (alongside Cleveland, Ponting, and Tewksbury). He was finally able to represent Canada at the Summer Olympics in 1992, where he finished sixth in the 100 m butterfly and won a bronze medal in the 4x100 m medley relay with Cleveland, Ponting, Tewksbury, and Stephen Clarke. After several successes at the 1993 World Cup, he retired from active competition, took up a career in real estate, and was inducted into the Ontario Aquatic Hall of Fame in 1997. He holds a degree in physical education from Comenius University in Bratislava.