Gallagher missed four games after sustaining a concussion in a 3-1 loss at the Carolina Hurricanes on Dec. 31. He wore a tinted visor when he returned in a 4-2 loss to the Edmonton Oilers on Jan. 9 and hasn't played in six games since.
"It's at the point where it's just really been day by day," Gallagher told the Canadiens website Tuesday. "I'm sure Thursday morning I'll be able to take the pregame skate and a decision will be made then. Until then, it's really day by day and just go from there."
The forward, who has 32 points (15 goals, 17 assists) in 41 games this season, practiced on the top line with Tomas Tatar and Phillip Danault.
"I've been headache-free since some point during the bye week (Jan. 19-26), so it's been a little while now. The headaches were there for a few days, and then after a couple of days it would come and go, come and go. But they've been gone for good since early in the bye week."