Gagne, 34, attended the Bruins' training camp. He did not play last season becuase he was not picked up by a team.
The last season the forward played was in 2012-13, when he scored five goals and six assists in 27 games with the Philadelphia Flyers.
The Bruins (1-3-0) travel to play the Detroit Red Wings (1-1-0) on Wednesday.
- Scooby Axson