Ruby gui for Desktop apps

MacRuby isn't a GUI. MacRuby is an implementation of Ruby that instead of C, makes use of modern tools like LLVM and an Objective-C implementation. This eventually allows it to communicate with Cocoa, with a "bridge".

It's a work in progress. Current version is 0.10; and the last source activity was 7 hours ago right at this moment, so I don't see the point of saying "no fresh news".

Regarding "almost no tutorial", have you checked the home page? The area called Documentation has many of tutorials, screencasts, recipes and these things.

There are several threads on stackoverflow about QT or WX and Ruby.

You might also want to check out Bowline. A Ruby desktop framework.