1. Login to your GOG.COM account
2. Enter this link: gog.com/redeem
3. Enter your code and complete Captcha task and click "Continue"
4. Confirm your redemption by clicking "Continue"
Gogcom
Activation guides for other platforms:
battlenet
bethesda
ea
epicgamesstore
other
playstation network
rockstar games
steam
ubisoft connect
xbox live