I wonder - does the GUI really belong to the OS? In all the examples I can give, the GUI is an application that runs on top of kernel and hosts the graphics calls. However all the systems I know have the non-gui applications which can run before.
So I'd say that GUI is not a part of the OS... Please tell if you agree or not
Arguably the OS includes the entire distribution, not just the kernel. MacOS ships the window server and the entire graphics stack, so the GUI is certainly part of MacOS, and so at least some OSes include a GUI.
So I'd say that GUI is not a part of the OS... Please tell if you agree or not