Gaucher played with Saint-Hyacinthe Gaulois in the
Quebec Junior AAA Hockey League (QMAAA) before he was selected 8th overall in the 2019 QMJHL Entry Draft by the
Quebec Remparts. In his draft-eligible season with the Remparts in
2021–22, Gaucher was awarded the
Michael Bossy Trophy as the QMJHL's Best Professional Prospect after scoring 31 goals and 26 assists for 57 points in 66 games. Ranked a mid-first round draft selection leading into the
2022 NHL entry draft by
NHL Central Scouting Bureau, Gaucher was selected 22nd overall by the Anaheim Ducks on July 7, 2022. Later that month, Gaucher was signed to his first NHL contract by the Ducks agreeing to a three-year, entry-level contract on July 29, 2022. Gaucher appeared in only 44 games for the Remparts in the
2022–23 season, but distinguished himself as a defensive forward with 22 goals and 24 assists, a 59.4% success rate at
face-offs, and a
plus–minus rating of +35, one of the best in the league. After the season, he was awarded the
Guy Carbonneau Trophy as the league's best defensive forward. The Remparts finished first in the regular season, earning the
Jean Rougeau Trophy. As QMJHL champions, the Remparts were the league's representatives at the
2023 Memorial Cup in
Kamloops, playing against the host
Blazers, the
Peterborough Petes, and the
Seattle Thunderbirds. Gaucher made his NHL debut on April 1, 2026, against the
San Jose Sharks. ==International play==