On my adventure of finding out why some GMod servers don’t list I have come to the conclusion that it is effected by the IP.
From what I have gathered GMod uses the steam server list which utilise some kind of GeoIP to display servers relevant to your area. If my hypotheses is correct then by updating the record it should display the server on the list, there is only one problem I don’t know which GeoIP list/service Valve utilises.
Any ideas on what service is in use?
Also if you want to know what has been tested I can post below keeping in mind all tests were done in an Australian locality.