I was just going through some of the portal pages and Chris has some extensive online help next to the various text/check boxes in the "?" circles. We can't hold Chris accountable for people not comprehending what they read on the portal. The online help did seem pretty accurate and well written.
It is not to hold Chris accountable, but some extra text above the form (that also explains what a gateway is and that setting up a one may not be what you want to do as a beginner) could be helpful. Still, there can be problems.
People will always make mistakes. I've seen some enter in their internal 192 or 10-net space IP as their commercial IP. Perhaps some sort of data checker can be put in place? That may help.
I hope the RFC1918 check is already made. If not it could be added. As I wrote before, there have been reasonable checks in place but people here have asked them to be removed because they wanted to do what the checks prevented. (like setting up a gateway with external address in net-44)
Rob