Gauthier was traded to the Anaheim Ducks by the Philadelphia Flyers on Jan. 8, 2024, for Jamie Drysdale and a second-round pick in the 2025 NHL Draft. He signed a three-year, entry-level contract with the Ducks on April 14 after helping Boston College advance to the 2024 NCAA championship game with 65 points (38 goals, 27 assists) in 41 games. He led the NCAA in game-winning goals (10), was second in points and tied for second in power-play goals (13), and named a Hobey Baker Award finalist, NCAA All-American and Hockey East First Team All-Star.

Gauthier, the No. 5 pick by the Flyers in the 2022 NHL Draft, solidified himself as a top NHL prospect prior to the trade when he was named the best forward at the 2024 IIHF World Junior Championship. He had 12 points (two goals, 10 assists) in seven games to help the United States win the gold medal. He scored 20 goals his first full NHL season (2024-25) and had his first hat trick and four-point game in Anaheim's 7-3 win against the Florida Panthers at Honda Center on Nov. 4, 2025. At 21 years and 289 days, Gauthier was the fourth-youngest player in Ducks history with a hat trick, following Leo Carlsson (18 years, 319 days on Nov. 10, 2023), Sam Steel (21 years, 51 days on March 26, 2019) and Isac Lundestrom (21 years, 115 days on March 1, 2021).

He was born in Sweden when his father, Sean Gauthier, was playing in Allsvenskan, the second-highest professional league in the country, but grew up in Arizona and played there until he was 10. That's when he moved to Michigan to find a higher level of competition, and he earned a spot with the USA Hockey National Team Development Program in 2020.

With the NTDP Under-18 team in 2021-22, Gauthier was second with 34 goals and fifth with 65 points. His nine points (three goals, six assists) in six games helped the United States finish second at the 2022 IIHF Under-18 World Championship.

Gauthier switched between left wing and center during his two seasons at the NTDP but was shifted to center starting in 2022-23 at Boston College. He scored in his NCAA debut, a 3-3 tie with Northeastern on Oct. 19. He was an alternate captain who had two goals and 10 assists to help the United States win gold at the 2024 IIHF World Junior Championship. He was named the 2024 Bob Johnson Award recipient by USA Hockey, recognizing excellence in international hockey competition.