St. Louis Cardinals

The St. Louis Cardinals, the team for which Gallen had rooted growing up,^[12] selected Gallen in the third round of the 2016 Major League Baseball draft.^[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. He started the 2017 season with the Palm Beach Cardinals and was later promoted to the Springfield Cardinals and Memphis Redbirds.^[13] He finished the 2017 season with a combined 10–8 record, a 2.93 ERA, and a 1.17 WHIP in 26 starts between all three clubs.^[14]

Miami Marlins

On December 14, 2017, the Cardinals traded Gallen along with Sandy Alcántara, Magneuris Sierra, and Daniel Castano to the Miami Marlins for Marcell Ozuna.^[15] Gallen later speculated that the Cardinals traded him to the Marlins because he was reluctant to attend a winter workout camp.^[16] He was a non-roster invitee to 2018 spring training.^[17] He spent the season with the New Orleans Baby Cakes, going 8–9 with a 3.65 ERA and a 1.47 WHIP in 25 starts.^[18] He returned to New Orleans to begin 2019.^[19]

On June 20, 2019, the Marlins promoted Gallen to the major leagues.^[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.^[21]

Arizona Diamondbacks

On July 31, 2019, the Marlins traded Gallen to the Arizona Diamondbacks for shortstop Jazz Chisholm Jr.^[22] In eight starts for Arizona, Gallen was 2–3 with a 2.89 ERA in 43+2⁄3 innings, striking out 53. In 2020, Gallen recorded 82 strikeouts in 72 innings. He also finished with an ERA of 2.75 and a 3–2 record.

Gallen returned to the Diamondbacks' rotation for 2022. Starting with an August 2 win over the Cleveland Guardians, he posted 34+1⁄3 consecutive scoreless innings before being named the National League (NL) Pitcher of the Month for August.^[23] The scoreless streak continued until a September 11 win over the Colorado Rockies, totaling 44+1⁄3 innings. It broke Brandon Webb's club record of 42, achieved in 2007, and was the seventh-longest in major league history. During the streak, Gallen produced six consecutive games started without allowing a run, tying the major league record.^[24] Gallen won the NL Player of the Week Award for the week of August 29 – September 4^[25] and the NL Pitcher of the Month Award for August.^[26] For the 2022 season, Gallen went 12–4 with a 2.54 ERA and 192 strikeouts in 184 innings. He finished fifth in NL Cy Young voting.^[27][28]

On January 13, 2023, Gallen agreed to a one-year, $5.6 million contract with the Diamondbacks, avoiding salary arbitration.^[29] He was named NL Player of the Week for the week of April 10–16.^[30] He was named to the 2023 MLB All-Star Game.^[31] Gallen pitched six no-hit innings in Game 5 of the 2023 World Series, but Arizona lost the series.^[32]