2008-09: Kevin Gagne made his QMJHL debut with Saint John as a 16-year-old and played for Canada Atlantic in the 2009 U17 World Hockey Challenge. In 48 games with the Sea Dogs he scored 3 goals with 3 assists and was +1 with 20 penalty minutes. Saint John finished third in the Atlantic Division and were swept by Cape Breton in the first round of the playoffs. Gagne was -1 with no points nor penalty minutes in four playoff games. He had 2 assists in five games for Canada Atlantic at the WHC.
2009-10: Gagne played 63 games for Saint John in his second QMJHL season. He scored 4 goals with 25 assists and was +39 with 40 penalty minutes. The Sea Dogs had the league's best record and won the Atlantic Division in the regular season but lost to second-place Moncton in the QMJHL Finals. Gagne appeared in five playoff games, suffering a season-ending knee injury, and was +2 with 1 assist and 2 penalty minutes.
2010-11: Gagne was an assistant captain and skated in 59 games for Saint John in his third season with the Sea Dogs. Overshadowed on a talented defense corps that featured first round picks Nathan Beaulieu (Montreal) and Simon Despres (Pittsburgh), he scored 6 goals with 26 assists and was +37 with 34 penalty minutes. The Sea Dogs again had the QMJHL's best record and defeated surprising Gatineau in the finals to reach the Memorial Cup. Gagne scored 2 goals with 5 assists and was +7 with 4 penalty minutes in 19 playoff games. The Sea Dogs defeated host Mississauga St. Michael's in the Memorial Cup final. Gagne scored 1 goal and was -2 with 2 penalty minutes in four Memorial Cup games.
2011-12: Gagne skated in all 68 games for Saint John in his fourth QMJHL season as the Sea Dogs again finished with the league's best record and rolled through the playoffs. He scored 9 goals with 26 assists and was +36 with 10 penalty minutes. Saint John won 16 of 17 games in the QMJHL playoffs and Gagne scored 2 goals with 13 assists and was +21 with 2 penalty minutes. The Sea Dogs came up short in their bid for a second Memorial Cup, falling to host Shawinigan in the semifinals (two days after beating the Cataractes in a round-robin game). Gagne had 2 assists and was +2 in four Memorial Cup games.
2012-13: Gagne signed a three-year entry-level contract with Anaheim in March 2013 and made his pro hockey debut with Ducks AHL affiliate Norfolk in April following an over-age season in the QMJHL. In five games with the Admirals he was -3 with 1 assist. Norfolk was fifth in the East Division; two points out of an AHL playoff spot. Gagne began the season in camp with the AHL's Hamilton Bulldogs but was cut and returned to Saint John. He skated in 31 games with the Sea Dogs before being obtained by Rimouski in a December trade. In 62 regular season games between the two teams he scored 9 goals with 26 assists and was +36 with 10 penalty minutes. The Oceanic finished second in the East Division and lost to Gatineau in a first round playoff series. Gagne scored 1 goal with 1 assist and was -6 in six playoff games.
Though small in stature, especially for a defenseman, Gagne plays a well-rounded two-way game. Not one to back down from a challenge, he makes a good first pass and works hard in all situations. On the powerplay, he has a great shot and moves the puck well. In his own zone, he helps keep the front of the net clear and works hard in the corners. However, his size is definitely a major limitation.
Picked up via free agency in March by the Ducks, Gagné played a limited amount with Norfolk, which was depleted with injuries and call ups. He will be in the mix for the Admirals blue line corps for the 2013-14 season.