In order to activate your GOG Key, please follow these steps:

1. First, claim the purchased product on your Kinguin account. 
2. Visit Gog.com and sign in to your existing account or create a new one if needed.
3. Click here to get access to the Redeem page.
4. Enter your game code.
5. Click the “Continue” button.
6. Press the “Redeem” button to finish the activation process.

Here you can check how to activate the GOG Key: