If you want to make cross-platform graphical applications with Ruby, you will need something else than DotNet.
I am not very aware of the capabilities of Ruby in terms of graphical application, as I use it mainly for parsers. However, I know that Ruby comes with the Tk library, a simple cross-platform library, although you probably do not want to use it, as it is a little limited and the applications made with do not look as good as those made with DotNet. You might want to use another library for that reason.
Other cross-platform GUI libraries include wxWidgets, GTK and Qt. It is up to you do determine if their Ruby port is sufficient for your needs though, as, like I said, I do not use Ruby for GUI applications. Note that Qt is a proprietary library and if you use it, you MUST make your application open-source and release it under the GPL. (Although it is hard to do otherwise with Ruby.) Otherwise, you will have to buy a license, which is rediculously expensive. ($4,000.00)
Also, if you want to do multimedia stuff, GUI libraries are usually not fast enough. For that, use the excellent cross-platform multimedia library SDL. I would not suggest that you use it if you are not experienced though, especially on a Linux machine because of how easy it is to crash X-Window.