Google Play Games Services overview

Official - Tutorial - Authentication with Google Play Games - Unity Forum

Google is ending support for multiplayer APIs in Play Games Services - Neowin

Google+ Games to get ax in favor of Google Play Games - CNET

Get Started With Google Play Games Services

Setting up Google Play Games Services for your Unity Android game, by James Brill

Play Games Services Google for Developers

Android Developers Blog: Enable next generation IDs for better Play Games Services support for all Google accounts

Google Introduces Cross-Platform Services For Its Play Platform - Game Informer

Google Play game services announced for Android and more – Destructoid

Play Games Services Google Play Console