If you do not have a Steam account, you will need to download the client from the official website, install it and then create your account. Once you are logged into your account, click on "Add game" in the bottom left corner and then on "Activate product on Steam". After a few clicks on "Next", you can finally enter your CD key, the game will automatically be added to your library and the download will begin.
Use your key on Epic Games launcher to download your game
Parkitect
To activate your game with Epic Games, log in to your account on their website, click on "Redeem Code" at the top right of the page, and then enter the game's CD key or code in the field provided. Click "Redeem" to verify the code and add the game to your Epic Games game library. You should now be able to download and play the game from your library.
CD key to use on the GOG GALAXY client to download your game.
Parkitect
To activate a game you download on GOG Galaxy, open the application on your computer, log in to your account, click on the menu icon, and then select "Game Codes". Enter the game code in the appropriate field and click "Continue". Once the code is verified, the game will automatically be added to your GOG Galaxy library where you can download and play it.
answering the message of 
