Gagne, 30, spent 10 years with the Flyers and was the team's longest-tenured player. Often injured, he appeared in just 58 games during the 2009-10 season after abdominal surgery, racking up 17 goals with 23 assists for 40 points.
He broke a foot in the first round of the Philadelphia's playoff run this spring, but returned much earlier than expected and recorded nine goals and three assists for 12 points in 19 post-season games to help the Flyers reach the Stanley Cup Finals.
"Simon Gagne played 10 seasons for the Flyers and was not only a good player for us, but also handled himself in a first class manner on and off the ice," said Flyers General Manager Paul Holmgren.
Walker, 30, spent one season with the Lightning, posted two goals and three assists in 66 games. He has also played for the Blues and Blackhawks in seven years in the league.