I’ve created a C++ wrapper for Steffen Itterheim’s GameKitHelper class that might be of use to some people. It doesn’t contain any of the multiplayer features, but it does do everything else - submitting scores and achievements, opening Game Center views, downloading Game Center data for populating custom views etc.
It’s available on my GitHub at https://github.com/SteveBarnegren/GameKitHelperCpp
There’s a basic explanation of how to use it in the readMe, or a more in-depth one on my blog at http://www.stevebarnegren.com/1/post/2014/01/game-kit-helper-for-c-cocos2d-x.html