The Flyers sent the Kings a conditional fourth-round pick in the 2013 draft that will improve to a third-round choice should they reach the playoffs with Gagne, who spent the first 10 years of his NHL career with Philadelphia.
Expected to be one of the top teams in the East, the Flyers have struggled to a 9-11-1 mark and were tied with Tampa Bay on 19 points, though the Lightning hold three games in hand to slot into the eighth and last playoff berth in the conference.
The Flyers have been in 16 of the last 17 Stanley Cup playoffs including two trips to the finals.
Gagne, 32, had five assists in 11 games with the Kings this season, but never really hit his stride after suffering an injury during the 2011-12 Stanley Cup championship season for Los Angeles.
The versatile forward has appeared in 772 career NHL games, scoring 283 goals and 586 points. (Reporting by Larry Fine in New York; Editing by Frank Pingue)