The modgui folder is responsible for a HTML interface, for web environments, and is documented at http://portalmod.com/ns/modgui
. If you want to try the web-based interfaces, you'll also need this project: https://github.com/portalmod/mod-ui
. But it might be a bit unfriendly, as currently it's not packed for desktop environments yet. The mod-ui project will need the mod-host software as host (look at portalmod's github profile), and communicate with it using sockets. If you would like to try that, I'll be glad to help you with any issues.
I can't tell about gtk or qt interfaces, I guess the widgets.* files are related to that.