After setting new career highs in goals (31), assists (34), and points (65) in 72 games, Gagnon was named to the WHL West First All-Star Team and was named the Western Conference nomination for the Brad Hornung Trophy as the WHL's most sportsmanlike player of the year,[5] an award he ultimately lost to Kris Russell of the Medicine Hat Tigers.

[6] On September 22, 2005, just prior to the start of his fourth full WHL season, Gagnon was named captain of the Thunderbirds.

Serving once again as the Thunderbirds' captain, Gagnon finished the season with career highs in goals (42), assists (38), and points (80) in only 59 games.

[13] In early August 2013, after spending his first five professional seasons in North America, Gagnon signed for two years with the Swedish Hockey League team HV71, joining Riley Holzapfel whom he played with in St. John's IceCaps.

He stayed at Lukko until January 25, 2017, and then moved to SC Bern of the National League (NL), where he inked a deal for the remainder of the 2016-17 season.