Gagnon started skating at the age of three, and a year later he took part in the first children's competitions. However, it took 15 years before Gagnon could achieve his first international success. At the Short Track World Championship in Beijing in 1993 , he won his first world championship title over 1000 meters as well as over 500 meters silver and over 3000 meters bronze. He also won gold in the overall standings in the end. A year later at the Short Track World Championship in Guildford in 1994 , he repeated the successes from the previous year and became world champion for the second time. He was also able to achieve this title in Cambridge with the team . In the same year Gagnon started for the first time at the Olympic Winter Games in Lillehammer . Over 1000 meters he won the bronze medal behind Kim Ki-hoon and Chae Ji-hoon . Over 500 meters and with the 5000 meter relay, he just missed the medal ranks with fourth place. A year later at the Short Track World Championships in 1995 in Gjøvik and Zoetermeer , he won a total of three gold medals. Another year later at the Short Track World Championships in 1996 in The Hague and Lake Placid , he won three gold and three silver medals. He could not build on this success at the 1997 Short Track World Championship . He only won gold here over 1,500 meters. On April 4, 1997, he set the world record over 1,000 meters with a time of 1.28.230. This record lasted two years until January 24, 1999. A year later in Vienna and Bormio he won a total of five gold medals. At the 1998 Winter Olympics in Nagano , he won gold with the 5,000 meter relay. At the Short Track World Championship in The Hague in 2000 , he was only able to achieve the world champion medal with the team. In 2001 at the World Championships in Jeonju and Nobeyama he was able to win gold again with the team after 2000, as well as gold over 1,500 meters as well as two silver medals and one bronze medal in the overall standings. At the 2002 Winter Olympics in Salt Lake City , he was the first time Olympic champion over 500 meters and with the 5,000 meter relay. He won bronze over 1,500 meters. In his actual parade discipline, he only reached 16th place. After the Olympics, he ended his active career.