Symptoms
When Google Checkout is used and attempts to access gcCallback.aspx page, the user is redirected to the storefront login page.
Cause
The AppConfig: GoogleCheckout.AuthenticateCallback is reversed in version 9 software (known bug) through MSx9.2.0.0 so set this to TRUE, and verify the /gcCallback.aspx page is now accessible on your site without authentication.
If it is still requiring authentication, then Basic Authentication is possibly being used and the gcCallback.aspx requires anonymous access. This is typical of some server hosts. Check with your server host if this is the case. If it is a required setting, then perform the following modification to the {root}/web.config file:
Solution
1) Backup your web.config file.
2) Add the code below to the end of the web.config file just before the closing configuration tag:
<location path="gcCallback.aspx">
<system.webServer>
<security>
<authentication>
<basicAuthentication enabled="false" />
</authentication>
</security>
</system.webServer>
</location>