Here a simple walk through on how to add your gmail account in Windows 8 Mail App.

In you Windows 8, access Settings from the Charms menu (move your mouse to the upper or lower right corner of your screen or in your keyboard, simply press Windows Key + C)

In Settings click Accounts

Under Accounts click Add an account

Choose Gmail…

Enter your Google account credentials and click connect then you’re done.

You may add other accounts by following same steps.