The person spoke on the condition of anonymity because the team has not announced the move made Wednesday.
The decision is regarded a formality. Two weeks ago, the Bruins announced Gagne was placed on their "suspended" list and would miss the remainder of the season following the death of his father in December.
Gagne is a two-time 40-goal-scorer with Philadelphia. The 34-year-old was attempting a comeback with Boston after missing all of last season.
He had three goals and an assist in 23 games with the Bruins this season.