Gagne tallied 23 assists and 26 points in 53 games during the 2020-21 season with the Muskegon Lumberjacks of the United States Hockey League. He also registered a franchise-record plus-35 plus/minus rating.
Gagne played for USHL Cedar Rapids in 2019-20 and recorded eight points (2 goals, 6 assists) in 38 contests.
Gagne is listed at 6-foot-3 and 207 pounds. He becomes the second defenseman the Lightning took in the 2021 Draft after selecting Roman Schmidt with their first pick of the draft in the Third Round (96thoverall).
A native of Bedford, New Hampshire, Gagne is committed to play at the University of New Hampshire in 2021-22.