The source of the issue may be a simple bug that has existed in all v9 that I have tested:
The setting for Authenticate Callback in the cart is actually REVERSED. This should be fixed in the next release (version after MSx9200)
Go to the callback URL and see if it wants authentication.
https://www.yoursite.com/gcCallback.aspx
So reverse the setting in the admin console and try the callback page again. You also need to verify the correct callback URL is set in the Google merchant account Settings - Integration, and that you are using XML method, on version 2.0 on that page.
Erik Sutton
AspDotNetStorefront Technical Support