Solution: I observed in the portal that my IP and hostname was listed for the Gateway. I deleted the IP and hit "Save." Within seconds, navigating to the page shows the new IP in the field.
Question: Does this mean the hostname feature in the portal doesn't dynamically update the Gateway IP addresses using Dynamic Hostnames - every n minutes?
If you go into the portal via the web interface and delete the IP address, then save the gateway record, the software immediately tries to resolve the hostname and populates the IP Address field if successful. If you save the gateway record and the IP Address field is already populated, it assumes that is the correct IP and ignores the hostname field.
There is also a cron job that runs once an hour on the portal. It selects all the gateways that have the hostname field filled in and tries to resolve the hostname, if it successfully resolves what is there it populates the IP Address field with what it gets back from the local resolver library, which may be cached, but it does honour the TTL.
Regards, Chris