When it said "GUI framework", I kind of expected a way to make cross-platform apps with their native UI controls, not a web view with an embedded PHP executable.
If it was better done and with more manpower, there would be absolutely no problem with people using it...
Edit: besides Window and Menu, of course
Also, for example Mac OS X has lots of little widgets like the share button.
Maybe someone could make a Bootstrap-like project for these kinds of things?