The Incoming Gateway setting is intended when you're using SmarterMail as an incoming gateway to offload work from the main primary server.
Since you are leveraging a front-end appliance an incoming gateway is not needed on your primary server. The spam front end will be acting as the incoming gateway. You will control the flow of mail to the incoming gateway by modifying your MX records.
No further configuration is required in SmarterMail. You may want to consider whitelisting the IP address of the front-end gateway for SMTP traffic under Settings -> Security -> Whitelist. This will ensure your gateway server does not get blocked by any IDS rules within SmarterMail.
I hope this helps clarify.
Technical Support Supervisor