I'd like to switch to GKGameSession (instead of the legacy GameCenter), but my app does a lot of auto-matchmaking. How do I do that with Sessions? Is it possible to create a game using the traditional MatchMaker View Controller, but then pass enough information for everyone to connect to the session?
For example, can I just pass the session identifier, or the URL?
You can't, there is no auto-matching. Other players join games by clicking on a sharing URL. You send the URL out using whatever means of communication you want, iMessage, email, other messenger service etc.
The GKGameSession API is pretty simple but lacks documentation. I've created a test rig which allows you to try it out.