The deal is worth $850,000 according to Sportsnet's Elliotte Friedman.
After being traded to the Red Wings from the Edmonton Oilers at the 2020 trade deadline, Gagner recorded one goal in six games before the NHL season was postponed. In total, Gagner had six goals and 13 points in 42 games during the 2019-20 season.
The 31-year-old veteran has posted 164 goals and 459 points over 13 seasons. The London, Ont., native has previously played for the Oilers, Arizona Coyotes, Philadelphia Flyers, Columbus Blue Jackets and Vancouver Canucks.