He started playing in his hometown Ottawa City Senior League with the Ottawa Aberdeens, Ottawa Royal Canadiens, and the Ottawa Grand Trunk Railroad from 1914-17. He then moved to the Quebec City League and played for Laval University, Quebec Sons of Ireland, and Quebec Montagnais from 1916-20. His last year of amateur hockey was with the Edmonton Eskimos of the Alberta Big Four League in the 1919-20 season.
Gagne started his professional hockey career with the Edmonton Eskimos of the Western Canada Hockey League in 1921. He would also play with the Regina Capitals. In 1926, he moved to the Montreal Canadiens of the National Hockey League. He would also play with the Detroit Falcons, Boston Bruins and Ottawa Senators. In 1932 he moved back to Edmonton to play four seasons in the minor pro North Western League.
Gagne retired in 1936. He played in 228 NHL games and scored 67 goals and 33 assists.