In 2023, he was named the starting pitcher for the National League in the All-Star Game and helped the Diamondbacks reach the 2023 World Series.

[4] At the age of 11, Gallen joined the Tri-state Arsenal Baseball Academy in New Jersey, where he served as a pitcher and second baseman.

[10][11] He made his professional debut with the Gulf Coast Cardinals, posting a 1.86 ERA with 15 strikeouts in 9+2⁄3 innings pitched.

[14] On December 14, 2017, the Cardinals traded Gallen along with Sandy Alcántara, Magneuris Sierra, and Daniel Castano to the Miami Marlins for Marcell Ozuna.

[20] He made his debut that night in a start versus the Cardinals, recording six strikeouts and giving up one earned run over five innings pitched.

During the streak, Gallen produced six consecutive games started without allowing a run, tying the major league record.

[27][28] On January 13, 2023, Gallen agreed to a one-year, $5.6 million contract with the Diamondbacks, avoiding salary arbitration.

On January 11, 2024, Gallen agreed to a one-year, $10 million contract with the Diamondbacks, avoiding salary arbitration.

[34] He made his return on June 29, when pitched six scoreless innings vs. the Oakland Athletics and only allowing one hit.