Shea Weber scored on the power play 1:24 into the second period for a 4-1 lead, and third-period goals 18 seconds apart by Gallagher (2:11) and Max Pacioretty (2:29) made it 6-1.
Mark Stone brought the Senators within 6-2 at 5:46, but Jacob de la Rose scored shorthanded at 7:08 and at even strength at 10:04 for an 8-2 lead. Victor Mete scored at 13:43 on the power play to make it 9-2.
Carey Price made 15 saves for the Canadiens, who finished the preseason 2-6-0 and open the regular season Oct. 5 against the Buffalo Sabres.
Craig Anderson stopped 24 of 30 shots, and Mike Condon seven of 10, for the Senators, who finished the preseason 3-3-0 and open the regular season Oct. 5 against the Washington Capitals.
