Sorry to post this here, but there isn't a Witcher 3 forum yet.

Anyway, I remember that for witcher 1 and 2 GOG provided a a backup copy for anyone who had the game on steam(?)

What are the chances for a same policy with W3? I was planning on buying W3 on GOG but I actually won a copy on steam, so good news/bad news eh? :P
Would really love to play my version through GOG

There are no plans for a Witcher 3 GOG backup as of yet. Might happen some day in the future though.

Up to yesterday, I would have told you that it currently won't be implemented, according to Witcher 3's official FAQ (check question 8). Today, though, CDPR announced that the retail version will contain a GOG code. At this point, nothing is certain.

Hm...this seems very ambiguous...
They're talking about a GOG code with the retail version, not steam version from what i understood

Says in the article that the box version will have a GOG code. I guess that's what you meant by "retail"?

Yes, that's what I mean. You will be able to play the game without it, but you'll also be able to redeem it via GOG. Yes, and like I said above, the official FAQ for the Witcher 3 mentioned:

Will I be able to redeem a backup copy of The Witcher 3: Wild Hunt on GOG.com?

The backup program for The Witcher 3: Wild Hunt is not planned any time soon, for sure not around release time. It was a one time action for The Witcher and The Witcher 2: Assassins of Kings. It might happen sometime in the future, but, currently, it's not in our plans.

Obviously, this is no longer true for the retail version, but who knows about versions from digital stores?

GOG has now a excellent restore function with can easily restore your previously installed and updated games.
Before reinstalling the System or GOG Galaxy simply copy the folder where you have all your GAMES and paste it somewhere safe. Once you have completed your re installation start the new GOG Galaxy to see if it works. Exit GOG Galaxy. Copy or move your backed up games folder back to your desired location.

Start the GOG Galaxy again. Go to library, choose the game you wish to restore Click the MORE button go to Manage and Import folder. Choose the game directory and let it do its thing.

To restore Witcher 3, 32 gig including updates it only took 250 MB of internet tragic.

Thank you GOG, we love you.

P.S. As far as I am aware GOG doesn't have could save function. So make sure you will back up your saves properly when you are backing up the game as well.