At Google I/O, Google announced Androids new Google Play game service. From what I remember and from what I've been reading online, it has already been released. Though, I haven't seen it on the Google Play store.
Is this something that exclusive to certain devices?
I have a rooted S3 if that matters.
End users don't need to install anything from their side. Google Play Services APK in your device would have already been updated to support Gaming (You can manually install/update Google Play Services framework if something's wrong). Just find a game which uses Game Services API and enjoy!
The new game features announced at I/O 2013 are enhancements to Google Play Services. Google Play Services is a framework that allows app developers to take advantage of standardized Google services like Maps, Google+, and the new Game services. It's up to app developers to implement changes and new features to these services, so you'll need a current version of Google Play Services and applications that take advance of the new stuff (Riptide GP was advertised as an app taking advantage of the new cloud save and achievement services, for example). Google Play Services is updated automatically in the background and is normally installed on Android devices by default (if they're allowed access to the Play Store by Google; your S3, in its stock configuration, is). If for some reason you're missing it, though, you can download it from the Play Store here.
While the Games Services are a background service that comes as part of the standard Google Services, Google have recently released a new Google Play Games app that centralizes your list of games into one launcher and (for games that support the new Game Services) also shows and syncs achievements between devices.
