The following widgets do not have an associated window. If you want to capture events, you'll have to use the EventBox. See the section on the EventBox widget.
GtkAlignment GtkArrow GtkBin GtkBox GtkButton GtkCheckButton GtkFixed GtkImage GtkLabel GtkMenuItem GtkNotebook GtkPaned GtkRadioButton GtkRange GtkScrolledWindow GtkSeparator GtkTable GtkToolbar GtkAspectFrame GtkFrame GtkVBox GtkHBox GtkVSeparator GtkHSeparator
We'll further our exploration of GTK by examining each widget in turn, creating a few simple methods to display them. Another good source is the set of programs avaiable in sample/test/ in the Ruby/Gtk distribution.