Gagne, who scored his fourth goal in the four games (all wins) since his heroic return from a broken foot, completed the comeback from a 3-0 first-period deficit and pushed Team Resilient into the Eastern Conference finals against the eighth-seeded Montreal Canadiens.
A gimpy Gagne isn't at full strength, but he said his foot was "getting better and better" with each game.
"The Stanley Cup is all about sacrifice . . . like Simon did tonight," Flyers coach Peter Laviolette said.
Amazingly, the Flyers, who needed a shoot-out win on the last day of the regular season just to sneak into the playoffs, will get the home-ice advantage as the seventh seed, the first time that has happened in NHL history.
The conference finals start Sunday night at the Wachovia Center. The series winner goes to the Stanley Cup Finals against Chicago or San Jose.
Registering their fourth straight win, the Flyers joined the 1942 Toronto Maple Leafs and the 1975 New York Islanders as the only NHL teams to win a playoff round after being in a 3-0 series hole. The 2004 Boston Red Sox are the only other team, in any sport, to make such a comeback.
"We've been resilient all year. Whether it was injuries or putting ourselves in a bad position in the standings," Richards said. "We have been through a lot together. Our mind-set was if you are going to go down, you are going to go down swinging."
Defenseman Chris Pronger said that the Flyers were proud to overcome a 3-0 series deficit, but that they were still on a mission.
"It's great to be a part of history, but we're eight wins away from where we want to be," he said.
Boston had been 16-0 in series in which it had won the first three games.
The Flyers advanced to the conference finals for the fourth time since 2000.
With the score tied, 3-3, both teams fired shots off the post in the final period - the Pronger and Boston's Milan Lucic (two goals).
Trailing by 3-0 in the opening period, the Flyers - in a microcosm of this series - roared from behind to tie the score heading into the third period.
"They started strong. They threw everything at us and got a couple of power-play goals," Richards said. "We just had to settle down and play hockey and be confident."
James van Riemsdyk scored late in the first period to calm the Flyers - "maybe we were a little nervous at the beginning," Scott Hartnell said. Hartnell and Danny Briere used deft moves around the net to produce second-period goals and tie the game, 3-3.
"We didn't have that same jump and we kind of backed off" in the second period, said Tuukka Rask, the Bruins' rookie goalie.
The Flyers appeared to take a 4-3 lead with 5:14 left in the second period after a wild scramble in front, one in which Richards, van Riemsdyk, and Arron Asham had whacks at the puck. The Flyers threw up their arms to signal a goal as Rask went down in a pileup.
The video appeared to show the puck inching across the goal line and Boston defenseman Dennis Wideman gloving it and swiping it out of the net.
The replay officials in Toronto disagreed, ruling that the puck did not cross the goal line.
Both teams had stressed the importance of getting off to a quick start and setting the tone. The Flyers did that in Games 5 and 6.
The Bruins did it in Game 7.
Two high-sticking penalties on the Flyers. Two Boston power-play goals.
Hartnell was trying to knock the puck out of midair, but instead hit defenseman Matt Hunwick and was sent to the penalty box. Eight seconds later, Michael Ryder pounced on a rebound and scored on a turnaround shot from the right circle, giving Boston a 1-0 lead after 5:27.
Less than four minutes after that goal, Danny Briere knocked off Wideman's helmet with a high stick as he battled behind the Boston net. About 1½ minutes later, Lucic converted a Wideman pass into another power-play goal.
The lead grew to 3-0 on a soft goal allowed by Michael Leighton with 5:50 left in the first period.
Lucic, on a two-on-one, beat Leighton to the stick side on a right-circle shot. It was Boston's first five-on-five goal in 178:46 - almost three full games.
Lucic's second goal of the night prompted Laviolette to call his time-out. As it turned out, it changed the momentum dramatically.
The Flyers got to within 3-1 when van Riemsdyk's soft shot deflected off Boston's Mark Stuart and past Rask with 2:48 left in the first period.
It's no coincidence that the Flyers started their winning streak the next game after David Krejci, Boston's playmaking enter, suffered a season-ending dislocated wrist when he received a heavy hit from Richards.
And it's no coincidence that they didn't lose a game with Gagne in the lineup.
"He's a world-class player," Laviolette said.
Gagne, who scored the game-winner in overtime in Game 4, triggered a world-class comeback.
"Relentless. Isn't that our slogan?" said Pronger, with a gap-toothed smile, when asked to describe the Flyers.
Contact staff writer Sam Carchidi at 215-854-5181 or firstname.lastname@example.org. Follow on Twitter at www.twitter.com/BroadStBull.