Gallen, 28, is 2-2 with a 5.24 ERA in four games this postseason.
Gallen posted a 17-9 record with a 3.47 ERA in 34 games during the regular season. He led the Diamondbacks in wins, strikeouts (220) and innings pitched (210).
He struck out 11 and allowed one run in six innings of a 6-3 victory over Texas on Aug. 22. He is 1-1 with a 3.57 ERA in three career appearances versus the Rangers.