The previous titles of the game have provided a backup copy of the game on GoG.com. Is this also true for "The Witcher 3"? Does buying the Game on e.g. Steam or Origin provide a key for GoG?
Checking on Steam, you cannot currently (as of 19/05/15) retrieve a CD key for the game. I cannot speak for Origin purchases, but it seems likely to be the same if utilizing the Origin infrastructure / client.
This might change in the future, but based on the previous systems (basically, entering CD keys into GoG here), it seems that it's not possible. Additionally, the interface linked above doesn't mentioned The Witcher 3.