Gallen (hamstring) threw a bullpen session Tuesday, Theo Mackie of The Arizona Republic reports.
Gallen began throwing off flat ground Saturday, and Tuesday marks the first time he's thrown off a mound since landing on the IL with a strained right hamstring. Manager Torey Lovullo said the 28-year-old righty is slated for another bullpen session Friday, after which he will likely progress to pitching sim games before returning to Arizona's rotation.