I was using Using Unity 2018.2 and (GPG) Google Play Games for Unity 0.9.50.
My signin flow first logs in the device with DeviceId, then trys to LinkGoogleAccount with ForceLink=false. If that fails, due to LinkAlreadyClaimed, it then trys to LinkGoogleAccount with ForceLink=true and using PlayGamesPlatform.Instance.GetAnotherServerAuthCode(true, (authCode) => {....
After each successful authentication I set the preferred type, so after LinkGoogleAccount succeeds once the type becomes LoginWithGoogleAccount with CA=false.
All of that was working good. Then I updated to Unity 2018.3.0f2....
All the sudden LinkGoogleAccount would start to authenicate and the app would hard crash. (I missed the connection for awhile because I had worked on a test project outside of Google signin after updating Unity to 2018.3.0f2. When I tried to implement that test project into the main project, with the LinkGoogleAccount auth flow, I noticed it was broken.)
I discovered my GPG version was very outdated. As of January 4, 2019 the current GPG version is 0.9.57
https://github.com/playgameservices/play-games-plugin-for-unity/blob/master/CHANGELOG.txt
If you are updating GPG first save a backup of your project, then follow the instructions from the GPG GitHub https://github.com/playgameservices/play-games-plugin-for-unity/blob/master/UPGRADING.txt
The most current GPG version .unitypackage file can be found after cloning/downloading from https://github.com/playgameservices/play-games-plugin-for-unity , then look in play-games-plugin-for-unity-master\current-build
After the previous version and folders have been removed per instructions, in Unity go to Assets -> Import Package -> Custom Package and find the "current build" .unitypackage file.
Once Unity has finished, you need to check that Window -> Google Play Games -> Setup -> Android Setup is still correct for your project. If not you need the info from Google Console from the initial GPG setup you did.
I hope this helps someone if they are experiencing strange/broken GPG behavior after Updating Unity.