In his third WHL season, Gallagher was invited to the Canadian junior team's World Junior selection camp in December 2010. Failing to make the roster, he was among the initial cuts. The following year, Gallagher made the team for the 2012 World Junior Championships, held in Alberta. In the last contest of the preliminary round, Gallagher was named Canada's player of the game after recording a game-high seven shots on goal in a 3–2 win against the United States. In the semifinal, Gallagher recorded a goal and two assists on Canada's last three goals, helping them get within one goal of erasing a 6–1 deficit against Russia. Canada went on to win their bronze medal game against Finland 4–0. Gallagher finished the tournament with six points (three goals and three assists), tying for fifth in team scoring, while leading Canada with 35 shots on goal. Gallagher was also a part of Team Canada in the 2016 IIHF World Championship. During the tournament, he scored 2 goals and 3 assists for a total of 5 points in ten games as Canada won gold.