You should check out this gem: Rugui. From the homepage:
RuGUI is a framework which aims to help building desktop applications. RuGUI was mostly inspired by the Ruby on Rails framework, taking most of its features from it.
RuGUI approach is to provide a MVC design for the application, separating business logic from application presentation, making it easy to maintain and evolve the application.
I've been experimenting with it - so no expert on RuGUI - but happy using it. The developers are open for improvement. It supports unit-testing and rspec (with basic support for gui-testing, a few days after asking them).
It only lacks good documentation, but if you know your way around in Gtk it's easy to catch up.
Check out these blogposts: