>How do you exit the Gnome desktop back to the shell session that started it?
>TIA

Well, there are a couple issues here. First, Gnome isn't a "desktop". Enlightenment, Sawfish (Sawmill), those are window managers ("desktops"). Secondly, there are a couple different ways that X could be started. You can either type startx from a shell, or configure your machine to boot to graphical mode (runlevel 5). If you boot directly into graphical mode, then there isn't a corresponding shell. X is running on virtual display 7.
Most Gnome desktops have a little foot in the bottom right hand corner. If you click on that, there should be a logout option. That will return you to wherever you came from (shell or XDM or GDM or KDM....). If you want to get to a console window from X, hit ctrl + shift + f1 (thru f6). Then shift + f7 to get back to X or the login screen.

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