A GUI can be used in Unix (in fact, several). Most power users don't use the GUI because it can be too limiting. The power of what you can do in Unix is at the command line, not using a GUI to click on buttons.
Of course, this could be said about most operating systems; the GUI is there for user convenience but most administrators use the command line for most of their work.
Probably just a UNIX based with a GUI.
It in UNIX the GUI is separate from the operating system itself. You can uninstall the GUI and still have a completely working UNIX system.
No.
X Windows
Yes
x windows
Several different versions of UNIX have GUI's or Graphical User Interfaces. The most common example of UNIX with an interface is Mac OS X.
Unix is an operating system that provides a Common User Interface (CUI) for controlling a computer. Thus CUI can run an application that provides a Graphical User Interface (GUI) which makes controlling what the computer does a little easier (e.g. GNOME - The GNU Object Model Environment).
A GUI operating system is really anything that uses graphics to control what a system does. GUI stands for "Graphic User Interface". So there for, MACOS, Windows, and the Main screen of Linux/Unix is considered a GUI, the operating system is the part of software that controls what the computer is doing.
If you are referring to the desktop as being a graphical user interface then Unix is capable of running without it. A GUI was a more recent addition to Unix, meaning that the earlier versions just used a command line and had no desktop environment at all. Today, you can install versions of Unix or Linux that do not have a graphical component at all, and several administrators (such as myself) prefer not to use a desktop.
It depends; some users may use a GUI while others prefer the command line (just like Windows, for example).
Because Linux was modeled after Unix, and designed to be a suitable free replacement. Windows is targeted at a slightly different market, and was primarily designed to be operated using a GUI, not the command line.