Hello James,
Thanks for the inquiry. I would setup the forwarding under the user profile for the account in SmarterMail. I would also complete the 'Reply To: field with the SmarterMail account of your choice, if different. Once the email is delivered to Gmail account you will then choose to reply, which will go back to the SmarterMail account it was sent from or you can choose to reply to all and then have the SmarterMail account that was entered in the Reply To field.
Please let me know if this helps.
Thanks.