The feisty rookie forward left Saturday night's game early in the third period after taking a hit from Flyers defenceman Luke Schenn.
Canadiens coach Michel Therrien said Gallagher was already "feeling a lot better," though he stressed that the club will follow league protocol and make sure he's fully recovered.
Under the NHL's new rules this year, a player who suffers a concussion no longer has to remain inactive for seven days, but must pass a series of tests.
Gallagher had a goal and an assist against the Flyers before the injury, as the Canadiens went on to a 4-1 win.
Forward Max Pacioretty, who also left Saturday night's game after getting cut by a skate as he drove to the net, was back at practice Sunday.