For just testing the port redirection and the camera, there's no need for a gateway because all the IP's being used in the test are in the current Class C sub-net. There should be no other IP's involved. I was trying to figure out a way to literally disable the gateway IP, but I guess the firmware authors didn't like either of my suggestions (same as router IP and 0.0.0.0).
Fine. Whatever it takes because the gateway IP is not being used for anything in this test. It's a reather unusual situation which I don't think the firmware authors every considered.