I think for right now, you'd have to use the C interop and link against something like GTK or other GUI library. I don't think SwiftUI specifically would ever work well cross-platform, it has so many Apple-isms.
There's also Swift/Win32, but it's MVC. You could also use the interop and use QT as well.
I don't think any of those are necessarily good solutions, but they are there.