Gagner, 29, signed with Vancouver as an unrestricted free agent on July 1, 2017, agreeing to a three-year, US$9.45-million contract.
The native of London, Ont., was coming off a career-high 50-point season with Columbus Blue Jackets, but mustered just 10 goals and 21 assists in 2017-18 with the Canucks.
In 770 career games with the Canucks, Blue Jackets, Edmonton Oilers, Arizona Coyotes and Philadelphia Flyers, Gagner has 152 goals and 281 assists.
"It’s not an easy decision," Vancouver head coach Travis Green told reporters after practice Monday. "We’ve said all along we’re going to have some tough decisions to make.
"We felt like it was the right decision right now with where our team’s at and where our organization’s at."
The move allows the rebuilding Canucks to keep younger forwards like Nikolay Goldobin and Tyler Motte with the NHL club as Vancouver prepares to open its regular season Wednesday at home against the Calgary Flames.
If Gagner clears waivers, the sixth overall pick by Edmonton at 2007 NHL draft will be assigned to the Utica Comets of the American Hockey League.
The Canucks also announced Monday that forward Darren Archibald has be put on waivers.