In order to redeem the Green Gift, please:

1. First, claim the purchased product on your Kinguin account.
2. Go to this website.
3. Log in or register a new account.
4. Enter your Green Gift in the code box and click "Apply".
5. You'll get a pop-up showing details about the product you are redeeming. Click the "Redeem" button to continue.
6. Upon purchase, you will be asked to log into your Epic Games account through an API which will grant you access to the game. 
7. You will be prompted to confirm the redemption of the game and on which account it will be redeemed. If this is not the correct account, please use the logout button and log in again with the correct account.
8. Once redeemed, your game will appear in your Epic Games account.

Here you can check how to activate the Green Gift in a different way: