Gagnon has made 18 appearances on Team Canada, making his first appearance at the 2001 Pan American Championships, when he earned a bronze medal in doubles with Francis Guillemette. Gagnon's greatest success internationally has been winning the 2007 Pan American Championships in Santiago, Chile, where he and François Viens defeated Americans Andy Hawthorne and Jason Samora in the final. Most recently, Gagnon was the silver medalist in Men's Doubles with Mike Green at the 2014 World Championships, reaching the final with a win over Americans Ben Croft and Tom Fuhrmann in the semi-finals before losing to Colombians Sebastian Franco and Alejandro Herrera in the final, 15-11, 15-5. Also in 2014, Gagnon was the bronze medalist at the 2014 Pan Am Championships in Santa Cruz, Bolivia, where he lost to Mexican Andree Parrilla, 15-10, 15-9. In 2012, Gagnon was part of the Canadian men's team that got silver at the World Championships, when he also played doubles with Green. In the 2003 Pan American Championships, Gagnon was a bronze medalist in singles, losing to American Jack Huczek in the semi-finals, resulting in a bronze medal. Gagnon was a bronze medalist at the 2010 World Championships, again losing to Huczek in the semi-finals. Gagnon won bronze at the 2009 World Games, as he defeated Mexican Alvaro Beltran, in the bronze medal match, 15-11, 12-15, 11-7. In that tournament, Gagnon lost to Huczek in the semi-finals, 3-15, 15-8, 11-7, which put him in the bronze medal match. Gagnon also earned bronze at the 2011 Pan Am Games by reaching the semi-finals, where he lost to Mexican Gilberto Meija, and bronze at the 2012 Pan American Championships, as lost to Fernando Rios of Ecuador in the semi-finals.
Canadian career
Gagnon won the 2014 Canadian Championships in both singles and doubles. He defeated Tim Landeryou in the singles final, 15-5, 13-15, 11-5, and with Samuel Murray beat Landeryou and Pedro Casto in the doubles final, 15-12, 15-2. Gagnon is the first Quebec born player to win the Canadian men's singles championship. The singles win came after Gagnon had been runner up at the 2012 and 2013 Canadian National Championships, losing to Mike Green both times. Gagnon won three previous Canadian Championships in doubles all with partner François Viens. They won in 2008, beating Green and Brian Istace in the final, and 2009, when they defeated Green and Eric Desrochers in the final. In 2012, Gagnon and Viens defeated Desrochers and Castro in the final, 15-8, 8-15, 11-9. Gagnon was the #1 men's player in Canada from June 22, 2010 to February 23, 2011, although he hadn't won a national championship in singles. But Gagnon did win the 2010 National Team Selection Event.
