The initial, pre-premiere retail PC batch of The Witcher 3: Wild Hunt needs to download a file from our servers to allow play. This file will either be downloaded automatically during installation process (via GOG Galaxy, optional DRM-free gaming platform of GOG.com) or you can choose to download it manually. It’s available at play.thewitcher.com.
Since we have no DRM in the game, this is a security measure we needed to incorporate so the game doesn’t leak during the production process. No worries, the file will not be big. Copies of The Witcher 3: Wild Hunt produced after launch will not contain this security feature.