I gave a talk on Google Play Game Services at the GDG DevFest Karlsruhe 2014. The talk was in German and I uploaded my presentation:
To show some sample code, I have also written a very simple game for my talk. The source code is on GitHub and you get the game from Google Play. If you’re interested in this subject and you want to discuss with me, then feel free to post a comment or to contact me on Google Plus or GitHub.