The short answer: no (because you said cross-platform).
The long answer: cross-platform GUIs are an age-old problem. Qt, GTK, wxWindows, Java AWT, Java Swing, XUL -- they all suffer from the same problem: the resulting GUI doesn't look native on every platform. Worse still, every platform has a slightly different look and feel, so even if you were somehow able to get a toolkit that looked native on every platform, you'd have to somehow code your app to feel native on each platform.
It comes down to a decision: do you want to minimise development effort and have a GUI that doesn't look and feel quite right on each platform, or do you want to maximise the user experience? If you choose the second option, you'll need to develop a common backend and a custom UI for each platform.
ruby is not a bad choice for your common backend.
