FWIW, I don't feel like there is any harm for the users to have access to the gateway landing page. They cannot do anything to your Gateway without authentication.
There is no way within Ignition to block the gateway web UI. Any block on port 8088 will kill everything.
Consider using a reverse proxy exposed to the internet and use rules within it to block the URL patterns that correspond to the gateway web UI.
You should also strongly consider not exposing the unencrypted port 8088 to the internet at all. Set up SSL, on the proxy perhaps, so all internet traffic is properly protected.
You could create a security zone called local host, with an identifier IP address of 127.0.0.7 then in your gateway security general settings add SecurityZones/localhost to the gateway config permissions. Then you’ll only be able to access the gateway config UI from the machine itself. (Or similarly adjust this for your local network access if you want that)
Someone could still reach the gateway landing page from external addresses, so if a vulnerability within Ignition is discovered that could still be a concern. But they wouldn’t be able to access any of the config even if they phished an account login so it does add a layer of security.