I opened a ticket and confirmed that the issue was actually that the gateway I am using was setting the "Received" header to an FQDN instead of an IP address. The current version of Xeams doesn't parse an FQDN, but the developers are planning to add this capability in the next release.  When I tested with a different gateway that sets the "Received" header to an IP address in my gateway.dat file, the inbound mail SPF was scored based on the senders IP address and the gateway was successfully ignored. 

However, I did identify an issue with the "Simulate" and "Analyze" buttons when reviewing emails from the "Message Repository".  The SPF records are calculated with the last hop as the sender's IP address, so it seems that the gateway.dat is not used in this scenario.  This behavior will be confusing for someone that is troubleshooting email delivery using these buttons.