Gallen (elbow) is scheduled to throw a bullpen session Tuesday, Alex Weiner of ArizonaSports.com reports.
Gallen is ready to take the next step in his recovery from right elbow inflammation, which caused him to land on the Diamondbacks' injured list July 12. He was moved to the 60-day IL on Aug. 3, so the earliest Gallen can return is Sept. 7, though the Diamondbacks will have a better sense of the right-hander's recovery timeline once he gets more bullpen sessions under his belt.