Gmail is not exactly email, with a variety of proprietary technologies which means you do not get the full experience when you use a generic email app.
Google has of course not released a Gmail app for Windows Phone, which is where MetroMail comes in.
The 3rd party app offers support for Gmail-specific features such as archiving, label management, inbox categories, and much more.
It is also brings instant live tile syncing between all your devices, spanning across both Windows 8 and Windows Phone 8.
The app is normally a paid application, due to the need to run servers to offer notifications for new email, but due to an issue with in-app purchases the developer is now offering the app, for free, with ads for a limited time.
See screen shots of the app below:
The app can be found for Windows 8 and Windows Phone 8, in the store here.