If you do not have the app already, by default then you can easily download it from the Microsoft App Store. It is available at free of cost. It is one of the basic modern apps of Windows 8. In the Windows 8 Mail app, open the charm bar by pressing Windows +C hotkeys and select settings. Further, in the settings, select the Account option to configure accounts associated with your Windows 8. Here, click on the option Add account and add your Gmail account. For further help contact Gmail Technical Support.