That's right, it works across iOS and Android, as well as with Web-based games (sorry, no Windows Phone, at least not yet). It debuted on a handful of games on Wednesday: World of Goo, Super Stickman Golf 2, Beach Buggy Blitz, Kingdom Rush, Eternity Warriors 2, and Osmos.
Unlike Apple's Game Center, Google Play Game Services offers backend support, rather than a standalone application.
To use the service, a gamer logs in -- lets say on their smartphone -- with their Google+ account. Once a game is saved, he could then move to the same game on his tablet and pick up where he left off. Cross-platform as it is, he could also move to an iPhone or iPad.
The company also launched APIs to help developers build multiplayer games. Google will handle the back-end connections necessary to keep the players connected over a wireless network.
Google isn't making use of these new features mandatory, though any developer launching a game on Google Play has access to them. Google lead product manager Greg Hartrell said:
We won't make it a mandatory exercise, or have any certification process around it. We create fantastic services that allow developers to create these great game experiences, and help promote their discovery, help retain their users and keep them engaged.