After attending the Maple Leafs' training camp, Gauthier joined the American Hockey League Toronto Marlies for the 2015–16 season.[23] He recorded his first professional point on October 21, 2016, against the Rochester Americans[24] and his first professional goal in a 3–1 win over the Lehigh Valley Phantoms.[25] He received a call-up by the Toronto Maple Leafs on March 19, 2016, and made his NHL debut on March 19, 2016, in a game against the Buffalo Sabres.[26] He played seven games with the Maple Leafs before being sent back to the AHL.