Hiya,
It looks like this use case is documented in the 'Complete the Google Login' section of the API documentation: https://fusionauth.io/docs/v1/tech/apis/identity-providers/google#complete-the-google-login
High level:
Developer does the Google login dance themselves Google returns a token Developer calls the IdP Login API as outlined in the above link.Hope that helps. I've also filed a PR against the documentation to highlight this functionality.