This is the current workaround for enabling MacOS login with Google.
The current version of pubspac.yaml generated by FF has a dependency to
google_sign_in: 6.1.5So, if you check the DART implementation for that specific version on GitHub, you can see that the implementation for 6.1.5 does not have a platform implementation for macos
https://github.com/flutter/packages/blob/google_sign_in-v6.1.5/packages/google_sign_in/google_sign_in/pubspec.yaml#L16-L20
But when you make a local modification to the pubspec.yaml and upgrade it to the latest one 6.2.1 which has a MacOS platform implementation
https://github.com/flutter/packages/blob/google_sign_in-v6.2.1/packages/google_sign_in/google_sign_in/pubspec.yaml#L19-L20
Then everything works, but you also need to upgrade other packages, which transitively depend on the google_sign_in
The last thing is to upgrade the MacOS Deployment target from 10.14 to 10.15 because google_sign_in requires it.
Changes should be made in ./macos/Podfile and in the ./macos/Runner.xcodeproj
