I would like to thank you all for your great answers.
It's so nice to get all these reasons without even asking for them :-)

I have decided to start with GTK on a Linux machine.
When I've learned enough to make useful programs, I'm sure the Windows
version is released :)

Where can I find API and tutorials on GTK?
What is Ruby-specific? Is there some syntax used in the API that has to be
written in another way in Ruby. I mean basic things like method names,
parameters etc.

Henrik