Worse than that, they were … friendly.
Gallagher (of the Canadiens) and Marchand (from the Bruins) happily chirped at each other on Twitter. They laughed, joked, and constantly needled one another, a bromance that took root almost from the moment they met at a brief training camp in Prague prior to the 16-day tournament in Russia.
In the end, Gallagher and Marchand would celebrate Canada's gold medal in Moscow after a 2-0 win over Finland in the championship game, a memory they can share whenever they're exchanging NHL face washes or introducing each other's mug to the corner glass.
"It was … interesting," Gallagher said with a laugh of his most unlikely friendship with Marchand, from his summer home near Vancouver.
Video: MTL@WSH: Gallagher cashes in for PPG on rebound
Each forward is cut from the same sheet of coarse sandpaper, a crease-crowding, opponent-antagonizing, game-changing nightmare for his foes, loved by fans of his team and detested by pretty much everyone else.
They've had their share of head-to-head battles on NHL ice, laying each other out, feeling each other's stick across shoulder blades and ankles. So here they were in the Czech Republic and then Russia, instant friends who each quickly learned that the other guy is actually - surprise! - a decent human being.
"I'd never met Marchy before the tournament," Gallagher said. "We walked in that first day in Prague and our stalls were right beside each other. We were kinda forced to talk… and we got along great. As tough as it is to say, I probably hung around Marchy more than anybody else for the course of the tournament."
At every opportunity, Gallagher said, he, Marchand and teammate Max Domi, of the Arizona Coyotes, would wage battle on the team's ping pong table, as competitive at table tennis as in the NHL's trenches.
"Marchy is obviously a very special player for what he does and what he contributes to a team," Gallagher said.
And then he laughed.
"But certainly once we get back to playing against each other this fall, I'm sure I'll get back to hating him pretty quickly. Don't worry, with the way he plays, it won't take too long for him to get back on my hated list.
"He's a very good player, a very competitive player, and he certainly gets under your skin," Gallagher said, summarizing very neatly how he himself is viewed by Canadiens opponents.
"I've always had fun playing against Marchy. He seems to bring out the best in me. We like competing against each other because we're so similar. It's just great to have that rivalry between the two of us."
Playing a valuable role for Canada in its World Championship victory - he had two goals and three assists in 10 games - was a bit of salve for Gallagher, who didn't enjoy a banner NHL season for the Canadiens, who tattered and frayed like a weather-beaten flag after some promising early-season success.
The 24-year-old missed 17 games from Nov. 25 to Dec. 29 with broken fingers, then another dozen from March 8-31 with a bad knee, having sprained a medial collateral ligament. He returned for the final four games of the season, the Canadiens long out of the playoff picture by then.
Gallagher had 19 goals with 21 assists in 53 games, giving him 77 goals and 79 assists in 260 career NHL games with Montreal.
Video: DET@BOS: Marchand rips home one-timer from Krug
"Missing another month of hockey, after the broken fingers healed (wasn't good)," he said. "But I was happy when I came back and had the chance to play a few more games, then go to Russia and win a gold medal, which was a really positive experience. As tough as this year was, it was nice to go over there and end on a positive note."
The golden achievement is celebrated in his offseason home. Gallagher has a framed game jersey on a wall above a table that holds, among other souvenirs, his diamond-studded World Championship ring from Hockey Canada. Frames nearby contain his 2012 World Junior Championship jersey and a Canadiens sweater from his 2012-13 rookie season, hung beside two teammate-autographed sticks from his international tournaments with his 2012 bronze medal and 2016 gold hanging from them.
A beast for fitness, Gallagher is already training hard for next season, having taken a very short break following last season to play golf in Arizona. He returned home to train for three weeks before the World Championship, then gave himself the luxury of two rest days upon his return from Russia before getting back into the gym.
He's also deep in planning for his second annual charity softball classic, Aug. 13 at Vancouver's historic Nat Bailey Stadium. Fellow NHL players and guest celebrities will help Gallagher raise money and awareness for Shriners Hospitals for Children - Canada, the Montreal pediatric health-care facility with which he's aligned and where he volunteers his time for star-struck young patients.
Gallagher and Canadiens defenseman Nathan Beaulieu, a good friend, expect to be at Rogers Centre in Toronto next weekend for the MLB series between the Toronto Blue Jays and Seattle Mariners, having been invited to take batting practice with the home team.
"I took BP with the Vancouver Canadians at Nat Bailey last year," Gallagher said of the Blue Jays' Northwest League affiliate. "I was so close to getting one out, hitting the top of the wall a few times, but I just didn't have the power. Hopefully this year.
"It'll be a bit of a test," he said, laughing. "It's not something I'm overly confident about, but as long as Nate doesn't get one, I'll be good."
The two then will head to London, Ontario, to take part in former teammate Brandon Prust's charity golf tournament, then Gallagher will return for his softball game and to step up his training.
"The way my season ended, I've had my time off," he said. "It's time to get ready for next year."