Hello all, I’d really appreciate it if anyone could help me. So, the issue I’m having is: when I copy the packaged Android APK of my game directly onto my device, my game works just fine, including the Google Play Games sign-in, the game can greet me with my Google Play Game username. However, when I actually publish the game to Alpha release on the Play Store, and then download the game onto my device, then the game is unable to sign into Google Play Games. I have checked everything, license keys, SHA-1, etc. What really bothers me is the fact that everything works just fine when I just copy the APK directly onto my device but then it fails when I download it from the Play Store (the very same version that is working fine when copied directly bypassing the Play Store). Please, any help would be greatly appreciated. Thank you.
I had this issue the other day.
Google App Store re-signs your app when you create a release with them, and Google Play Game Services doesn’t have this new key.
Go to Release Management -> App Signing. Copy the SHA1 under “App Signing Certificate”. This is the one that google generates and resigns with. Further down this page is an “Upload Certificate” that is the one you have signed with.
Go to Game services -> Your game -> Game details. At the bottom should be a link to Api console project. Go there.
Go to Credentials. Create a new OAuth Client ID and paste in the SHA1 from earlier. There should be an OAuth Client ID already that matches your upload one.
You should have 2 (or 3 if google auto created a web client) OAuth Client IDs.
If all is done right, apps that you copy onto the device should work (and use one Client ID), and apps installed from the play store should also work (using the other Client ID)
Hi there pukekopictures, thank you so much for your kind assistance, I really appreciate it. I follow your instructions and yes, the game seems to be working now! 
Once again, thank you so much! 