The GUI was Intuition, Workbench is the graphical shell, or desktop as it is known as today.
I didn't know anyone who said "Go into Intuition and launch that application." :-)
Since GUI is a Graphical User Interface, and the Workbench was the "interface", that's what I called it.
Now, when "programming" graphical features, I used the "Intuition" functions to achieve Windows and menus, etc...
But the actual Graphical User Interface we used to interact with the Amiga was Workbench. And it used the Intuition to achieve that..
I'd say that Workbench is similar to the Windows GUI and Intuition is similar to the Windows API that is used to enable Windows.
IMHO..
desiv