Apportable's implementation of GameKit uses the Java bridge to call the Android version of Google Play Game Services. Here are some games that are using it:
There's also an implementation of the Google Play Games Services Objective-C API, again using the Java bridge to call the Android version. Here's an example of an app using that:
Google Play Game Services support is included in all paid Apportable plans, so if you want achievements/leaderboards/multiplayer and can't be bothered to bridge it yourself, just upgrade. There is also a GameCircle-powered version of GameKit for the Amazon Appstore.