Gagnon played for the Montreal Junior Canadiens in 1961-62 and 1962-63, But it took him 9 years as a Montreal Canadiens farmhand to get a chance with the NHL team. He played 4 games with the Canadiens in 1971-72. During the summer of 1972 he was traded to the expansion New York Islanders. Gagnon played one and a part seasons with them before being traded to the Chicago Blackhawks near the end of the 1973-74 season. He played with the Hawks and the Kansas City Scouts and retired in 1976.