After attending the Maple Leafs' training camp, Gauthier joined the American Hockey League Toronto Marlies for the 2015–16 season.
He recorded his first professional point on October 21, 2016, against the Rochester Americans and his first professional goal in a 3–1 win over the Lehigh Valley Phantoms.
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.
He played seven games with the Maple Leafs before being sent back to the AHL.
During those seven games, Gauthier recorded his first career NHL point on April 6, 2016, with an assist on Morgan Rielly's second period goal.

After attending the Maple Leafs training camp, Gauthier was assigned to the Toronto Marlies for the 2016–17 season.
After playing 16 games with the Marlies, Gauthier was recalled to the NHL on December 16, to replace an injured Martin Marincin.
Gauthier would eventually record his first career NHL goal on December 22, 2016, against the Colorado Avalanche.
Gauthier was subsequently recalled periodically throughout the season but played with the Marlies during their 2017 Calder Cup playoffs run.
While playing in the second-round against the Syracuse Crunch, Gauthier suffered an avulsion of the hamstring injury which required surgery, ending his season.
Although his projected recovery time was six months, he was back skating in less than three months.
As a result of the surgery, Gauthier lost some hamstring muscles and became a bit shorter.

Gauthier was invited to participate in the Toronto Maple Leafs training camp prior to the 2017–18 season, although he was reassigned to the Marlies.
On January 2, 2018, Gauthier was called-up to the Maple Leafs to play against the Tampa Bay Lightning, a 2–0 loss.
He was sent back down to the AHL on January 11 after playing five games in the NHL so that he could continue playing hockey while the Leafs were on a bye week.
Gauthier and fellow Marlies player Travis Dermott were recalled on January 15.
On January 23, after playing a total of nine games in the NHL during which he scored one goal, Gauthier was returned to the Marlies.
He competed with the Marlies during their 2018 Calder Cup playoffs run where he helped them clinch the Calder Cup.

On July 11, 2018, Gauthier signed a two-year, two-way contract extension with the Maple Leafs.
After making the team out of training camp, Gauthier stayed on the NHL roster for the entirety of the season.
Gauthier played 62 games for the Maple Leafs before suffering a foot injury in March which kept him out of two games.
He recovered in time to make his post-season debut with the Leafs, playing in all seven games during the first round against the Boston Bruins.

Although the 2019–20 season would eventually be delayed due to COVID-19, Gauthier started the season strong.
He partook in skating lessons with Leafs coach Barb Underhill and captain John Tavares spoke highly of his progress; "He’s making plays more, feeling good about finding the open man and what’s going on in the open ice, and getting to the net."
Upon the hiring of Sheldon Keefe on November 20, 2019, he experienced his first healthy scratch of the season after playing in 29 games.
When inquired about his choice, Keefe stated "[w]e've liked how Fred's played, he's done well but we're trying to find the right mix for our team & we've got to try some different things.
In the interim he's going to have to take a step back a little bit."
After sitting as a healthy scratch for four consecutive nights, Gauthier returned to the Leafs lineup to reach a new career-high in goals.
After attending the Maple Leafs' training camp, Gauthier joined the American Hockey League Toronto Marlies for the 2015–16 season. He recorded his first professional point on October 21, 2016, against the Rochester Americans and his first professional goal in a 3–1 win over the Lehigh Valley Phantoms. 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. He played seven games with the Maple Leafs before being sent back to the AHL. During those seven games, Gauthier recorded his first career NHL point on April 6, 2016, with an assist on Morgan Rielly's second period goal. 
After attending the Maple Leafs training camp, Gauthier was assigned to the Toronto Marlies for the 2016–17 season. After playing 16 games with the Marlies, Gauthier was recalled to the NHL on December 16, to replace an injured Martin Marincin. Gauthier would eventually record his first career NHL goal on December 22, 2016, against the Colorado Avalanche. Gauthier was subsequently recalled periodically throughout the season but played with the Marlies during their 2017 Calder Cup playoffs run. While playing in the second-round against the Syracuse Crunch, Gauthier suffered an avulsion of the hamstring injury which required surgery, ending his season. Although his projected recovery time was six months, he was back skating in less than three months. As a result of the surgery, Gauthier lost some hamstring muscles and became a bit shorter.
Gauthier was invited to participate in the Toronto Maple Leafs training camp prior to the 2017–18 season, although he was reassigned to the Marlies. On January 2, 2018, Gauthier was called-up to the Maple Leafs to play against the Tampa Bay Lightning, a 2–0 loss. He was sent back down to the AHL on January 11 after playing five games in the NHL so that he could continue playing hockey while the Leafs were on a bye week. Gauthier and fellow Marlies player Travis Dermott were recalled on January 15. On January 23, after playing a total of nine games in the NHL during which he scored one goal, Gauthier was returned to the Marlies. He competed with the Marlies during their 2018 Calder Cup playoffs run where he helped them clinch the Calder Cup.
On July 11, 2018, Gauthier signed a two-year, two-way contract extension with the Maple Leafs. After making the team out of training camp, Gauthier stayed on the NHL roster for the entirety of the season. Gauthier played 62 games for the Maple Leafs before suffering a foot injury in March which kept him out of two games. He recovered in time to make his post-season debut with the Leafs, playing in all seven games during the first round against the Boston Bruins.
Although the 2019–20 season would eventually be delayed due to COVID-19, Gauthier started the season strong. He partook in skating lessons with Leafs coach Barb Underhill and captain John Tavares spoke highly of his progress; "He’s making plays more, feeling good about finding the open man and what’s going on in the open ice, and getting to the net." Upon the hiring of Sheldon Keefe on November 20, 2019, he experienced his first healthy scratch of the season after playing in 29 games. When inquired about his choice, Keefe stated "[w]e've liked how Fred's played, he's done well but we're trying to find the right mix for our team & we've got to try some different things. In the interim he's going to have to take a step back a little bit." After sitting as a healthy scratch for four consecutive nights, Gauthier returned to the Leafs lineup to reach a new career-high in goals.
Frédérik Gauthier