I bought the physical box for the Gwent cards that come with it but don't need the actual DLC, so selling it to get back some of the cost. According to the card, you can redeem it via GOG, Steam or Origin.
This is a code for DLC which requires you to have the base game. Blood and Wine retails for £16, I believe the price to be fair.
No longer for sale. Will give away for free on HUKD.
After payment, I will either send the code via message or can post the card physically.