I don't think it was too similar, TBH. Apple never took MacRuby as seriously as Microsoft took VB6, and it hadn't even had a 1.0 release when the single developer left Apple to work on RubyMotion.

I do agree it'd be interesting to have a GUI designer for Ruby. Does QML paired with QtRuby work?

In the distant past I had a book about FXRuby, but never used it much, and don't think it had a UI designer - it was just bindings to Fox Toolkit, which is lightweight, but not as well maintained as Qt or Gtk.