Gagnon started his Olympic career in 1994, when had already won the 1993 World Championships. He won a bronze in the 1000 m event. Four years later, in Nagano, Japan, Gagnon won a gold medal with the Canadian relay team. The 2002 Salt Lake City Games proved to be Gagnon's best Olympics, with a total of three medals. A bronze in the inaugural 1500 m event, and two golds; in the 500 m and again as a part of the relay team.
Winning a total of five medals in three consecutive Winter Games made him the most decorated athlete in Canadian Winter Olympic history.
Gagnon won his World Championships in 1993, 1994, 1996 and 1998. In addition, he finished 2nd twice, and third once.