Consider viewing this post on my new blog:http://adrian-dawid.me/gulp/c++/2014/08/18/google-play-cocos2d-x-tutorial.html, I will post any updated information only there.
This is a short tutorial how to use Google Play Games Services with cocos2d-x. It describes how you add the library to your project. Google Play Games is a really handy tool for mobile game development, but there is almost no documentation how to use it with engines like Cocos2d-X, that are using the NDK and are designed to be cross platform compatible. I am using the JNI Bridge between C++ and Java here to use all the java functionality. There is also a c++ sdk, but it is also not easy to integrate into cocos2d-x too and has not the full feature-set of the java sdk.
Adrian Dawid 