Google sign in signed apk not working

I had the same problem. I think I found out that Google doesn't allow you to have one certification for both the debug and the release apk of your app. You need to choose to either get the certificate only for one of them. Please correct me if I am wrong.

What I did was to input the SHA1 fingerprint credentials for my release key and not my debug key on this link here

Afterwards, my released apk worked and not my debug key.

As I understand, you have provided the debug SHA1 in the developer console, then you signed the apk and the SHA1 changed. If this is the case try the following you should obtain the release SHA1 from the keystore and replace the old SHA with that.

1. Open terminal and change the directory to JDK bin directory. Include your installed JDK version inside the path, for me it was - jdk1.8.0_101 (type javac -version to get the Java version) :

Mac

    cd /Library/Java/JavaVirtualMachines/<your_JDK_version>.jdk/Contents/Home/bin

Windows

    cd C:\Program Files\Java\your_JDK_version\bin 

2. Use keytool to obtain the release SHA1 :

    keytool -list -v -keystore <keystore_name> -alias <alias_name>

3. Go to your project's credentials page and replace the SHA1 to your keystore's release SHA1.