Nice tutorial. I'll have to take some time to work through it at some point.
I'm a C/C++ programmer by trade, but I love learning new programming languages. Ruby and Haskell are two I'm currently focusing on.
I've been messing with Ruby on and off for a couple of years now, but so far I haven't done much more than terminal based scripts.
I've used QT, wxWidgets and GTK with C++ and Python. But I haven't tried any of their Ruby bindings yet, so your GTK tutorial will be extremely handy. Thanks!