I'm using GameKit in my game, which is built both as a Universal iOS app and a Mac OSX app. As far as I can tell, there's no way to allow Mac users to play against iOS users, because each of the two are sandboxed into their respective app stores: Mac and iOS apps must each have their own (unique) Bundle IDs.
I tried adding both the Mac and iOS apps to the same Game Center "group." I am successfully able to see the same leaderboards from both the Mac and iOS app. Unfortunately, GKTurnBasedMatch loadMatchesWithCompletionHandler returns different matches for the same user on different platforms, and users from Mac cannot seem to be matched with iOS (and vice-versa).
Is there any way to enable truly cross-platform support with GameKit, or am I doomed to only allow Mac<->Mac and iOS<->iOS gameplay?
As of iOS 6.1, GameKit connects devices on their respective platforms only. I asked about this at WWDC '13 and was given that answer by an Apple engineer.