This tutorial gives an introduction to GUI programming using the Qt toolkit, Ruby, and qt4-qtruby. It is simply a port of Nokia's fine Qt Tutorial. Therefore, most of the text after this paragraph comes straight from their tutorial. I have modified it as necessary to make it make sense with the Ruby bindings, and have rearranged or rewritten a few parts to make them (I hope) a bit clearer.
This tutorial doesn't cover everything; the emphasis is on teaching the programming philosophy of GUI programming, and Qt's features are introduced as needed. Some commonly used features are never used in this tutorial.
Chapter one starts with a minimal "Hello world" program and the following chapters introduce new concepts. By Chapter 14, the "Hello world" program from Chapter 1 will have turned into a 448-line game.
If you're completely new to Qt, you might want to read How to Learn Qt if you haven't already done so. Keep in mind that like most documents about Qt, it is very C++ oriented.
This little game doesn't look much like a modern GUI application. It uses some GUI techniques, but after you've worked through it, I recommend checking out the Application example, which presents a small GUI application with menus, tool bars, a status bar, and so on. There are quite a few very educational Examples. They are all written in C++, but are easy enough to follow since the point of them is to demonstrate usage of the Qt library, not C++ features.