Call of Juarez Gunslinger for PC is a Steamworks game, which means that it can only be ran through Steam. To be able to play the game you will need to register for a (free) Steam account and download the Steam client.
There are three different ways to acquire/unlock the game:
For the Steam Store
If you have purchased the game directly through Steam, the game will automatically appear in your games library after the purchase has been completed. Click on Install to start the download of your game (approximately 4.7GB), and upon completion you can click Play to start your game.
There is no need to activate your game or enter a CD key.
Ubisoft Store and other digital retailers
After purchasing the game from the Ubisoft Store you will receive a confirmation email containing your Steam activation code in the format of XXXXX-XXXXX-XXXXX. To activate this code you need to install Steam and follow the steps below:
1. Launch Steam and log into your Steam account.
2. Click the Games Menu.
3. Choose Activate a Product on Steam...
4. Follow the onscreen instructions to complete the process.
Retail boxed game
Insert the game disc into your DVD drive and follow the on-screen prompts.