Gallen (elbow) completed a 33-pitch bullpen session in Arizona on Tuesday, Alex Weiner of ArizonaSports.com reports.
Gallen was able to complete the throwing session without any issues, successfully taking the next step in his recovery from right elbow inflammation. The right-hander isn't eligible to be activated from the 60-day injured list until Sept. 7, so the Diamondbacks may opt to have the 31-year-old toss at least one more bullpen session before organizing a minor-league rehab assignment.