We have a similar scenario where we use a routing rule to redirect the incoming message to a different non-domain address.
Having said that, none of our staff (myself included) have the Gmail service enabled on our workspace accounts. So we have a recipient address map setup that redirects all messages to our email accounts at a different domain. I just tested this, and the redirect from an internal account (same domain) worked fine.
The settings on the address map are simply 'all incoming messages' and 'Also route to original destination'. It just works.
If you were testing perhaps you needed to wait longer for the settings to fully propigate?