Gears of War 4 is a Play Anywhere title and is available through the Windows 10 Store app under your Microsoft acount. Once you redeem the code for Gears of War 4 on http://redeem.geforce.com, the game will be tied to your Microsoft account you provided during the redemption process.
Note: Gears of War 4 requires the Windows 10 Anniversary Update released in August of 2016. If you have not yet updated your Windows 10 to the Anniversary Update, please do this first. If your PC is on an older Windows 10 build, you will not be able to download Gears of War 4 from the Store app. For more info visit: https://support.microsoft.com/en-us/help/12387/windows-10-update-history
To download Gears of War 4, follow the steps below:
1) Click on your Windows key on your keyboard to bring up search and type "store"
2) The first result should be the Store app. Click on the icon to open the Store.
3) Once Store app is open, sign in to the store by clicking on the profile icon next to the Store search bar. Then select Sign in. If you are already signed in, you may skip to step 6.
4) Choose Microsoft account
5) Input your Microsoft account log in information.
6) Once you are signed in, click on the Profile icon and choose My Library
7) Gears of War 4 should appear under the Games section. Click on the down arrow to begin the download process.