If all you want to do is make dialog boxes, consider zenity. This is quite a flexible tool, although limited to dialog-type tasks.
If you want to implement a more completed GUI, it's going to get more complicated. Tk is a fairly easy-to-use GUI toolkit, and has bindings to several languages. Probably the easiest to work with (if you don't already know one of the alternatives) it TCL. TCL is a nice little language - there is not a lot of syntax to learn, and it turns out to be quite expressive. Using Tk from a TCL program is what is referred to with the name TCL/Tk.
If you install Tk, you will get a program called wish, which is a TCL interpretter with the Tk features available. The Tk package in Ubuntu comes with a whole bundle of examples, which you can read and learn from. Of you use another distro, YMMV.
For example, I find the file /usr/share/doc/tk8.4/examples/widget, and can launch it like this:
Code:
wish /usr/share/doc/tk8.4/examples/widget
This is a demo program which will launch all the other demo programs, and show you the code.
Note that the syntax of TCL is not the same as shellscript - it is a different language. Also note that Tk doesn't tend to look as nice as a lot of other GUI toolkits.