For a GUI in a limited sense you can use TIC-80 with its built-in Janet support. It can export to stand-alone binaries too, but I did not try that (and I mostly used TIC-80 with Fennel, not Janet).

Obviously I understand most of the time someone wants to make a GUI they are not thinking about something like TIC-80. I agree a more complete GUI library would be nice.

But if nothing else it may be worth mentioning that TIC-80 is a fun and easy way to install and try Janet. Even the Android version comes with Janet and a built-in code editor. Just run "new janet" in the TIC-80 console and then press F1 (on Android TIC-80 by default uses its own virtual keyboard that has all the keys it needs) to open the code editor.

Problem with the Android version is how ... to access the files it saves (or exports to). They seem to end up in the app-specific data directory that no other app can read on newer versions of Android.