I think you might be able to build the old GUI from source and then copy it inside your gapped environment and run it there, but @hatch would have to let you know how to do that, as I’ve never done it before.
If you just want to be able to change config on your applications through a GUI you can use Juju Lens, which has both a web application and desktop application which you can use without having to deploy anything or change your controller at all. It’s super easy to try out.