You have to use 3rd party apps to get Google Music outside the browser, but it's still essentially just using the regular web page with the addition of being able to use the media keys.
You can use the media keys with Google Play Music in a browser, at least in Chrome. No third-party app required.
I could be wrong, but I don't think those work globally (without an addon). Or has that changed recently?
It requires a bundled by default, Google Music app. If you disable or delete it, the media keys will stop working. Here is the download link for the app :