Claim the GOG.com digital backup copy of the game at www.gog.com/witcherbackup
Please note! GOG.com copies do not come equipped with game keys as the only use for a game key is to be able to claim a gog.com backup copy of the game to have a digital backup on gog.com in case something happened to one's non-gog.com copy of the game.
If you will find issues with redeeming the code, please contact GOG.com Support. In attachement provide a photo of the game code with your email adress/GOG username written on a sheet of paper.