GPGS achievement and Leaderboard Not Working?
Make sure you have followed this step
From Unity3d, click on Google Play Games > Android Setup, then change the ApplicationID.
Make sure you download SDK for Google Play Games, check at your Android SDK
If you still cannot login, I can assume this bug at GPGS for unity, you can change it manually
go to Assets\Plugins\Android\MainLibProj\AndroidManifest.xml, open this file and just replace your applicationID there.