For your information:
GPGS API update: Less clicks for users to sign in.
"With this new API model for Play Games, perhaps one of the biggest changes is that the players will no longer have to sign into their accounts every single time they open a game, but instead, will only be required to sign in once per account. This should help players spend less time signing into Google Play Games services and more time getting to the games they’re trying to play. In addition, Google+ will no longer need to be a part of the Google Play Games account process, so new play games users won’t need to “upgrade their account to Google+” to use the play games services."