The team announced the roster move Saturday morning.
Gaulthier, 20, has yet to suit up for an NHL game.
He has registered five goals and 11 assists for 16 points in 52 games with the Maple Leafs' AHL affiliate Toronto Marlies this season. Gaulthier was a first-round pick by the Maple Leafs, taken 21st overall in 2013.
Leivo, 22, has played nine games with the big club this season, tallying four goals in nine games. He has 14 goals and 26 assists for 40 points in 45 games with the Marlies.
He has a total of 25 career NHL games to his name since being drafted by Toronto in the third round (86th overall) in 2011.