During the 2010 playoffs, Gallagher was selected as the Male Youth Athlete of the Year in his hometown of Delta, British Columbia. In anticipation of the 2010 NHL Entry Draft, he was ranked 152nd among North American skaters eligible to be selected by the NHL Central Scouting Bureau. Gallagher was selected 147th overall by the Montreal Canadiens. Returning to junior following his first training camp with the Canadiens, he recorded WHL career highs in 2010–11. On February 28, 2011, he was named WHL Player of the Week after recording ten points (five goals and five assists) in three games. In March 2011, Gallagher was sidelined with a head injury, sustained after receiving a hit in a game against the Portland Winterhawks. Ranking first on the Giants and eighth among all WHL scorers with 91 points over 66 games, he was named to the WHL West First All-Star Team. His 44 goals were just four shy of Evander Kane's single-season team record.