Gregory joins defensive tackle Javon Hargrave (hamstring) and Elijah Mitchell (knee) in the "questionable" category for the Christmas night matchup.
In addition to these uncertainties, the 49ers have ruled out defensive tackle Arik Armstead (foot, knee), linebacker Oren Burks (knee), tight end Ross Dwelley (ankle), and wide receiver Jauan Jennings (concussion) for the upcoming game.
The 49ers and Ravens are set to kick off at Levi's Stadium on Monday at 5:15 p.m. PT. The team's depth on the defensive line will be tested as they navigate through these injuries in a critical matchup.