Below code I am using for Google login.I added google-services.json file in the app folder.I am using classpath \'com.google.gms:google-services:2.0.0
I faced the same issue with my React Native project.
I had to generate SHA-1 key using debug.keystore in android/app folder.
Run the below command in the terminal after changing your directory to cd C:\Program Files\Java\jdk1.8.0_251\bin. (You may have a different JDK version).
keytool -list -v -keystore "D:\{ProjectName}\android\app\debug.keystore" -alias androiddebugkey -storepass android -keypass android