I believe that in time (though probably not right on release date) we will be able to register our physical edition on GoG and have a digital backup there.
I expect that GoG backup copy to include:
- artwork
- wallpapers
- avatars
- bonus videos
- world Map
- digital soundtrack (the standard tracks)
I expect it NOT to include the other GoG/pre-order bonuses, such as:
- the digital artbook
- the 4 bonus soundtrack tracks
- (beta) access to The Witcher Adventure Game
- free copy of Neverwinter Nights
- The Witcher: House of Glass digital comic book
(Then again, if they REALLY want to push GoG Galaxy, they may even decide to give you that backup copy on GoG right on release date...)