in Ubuntu Intrepid we have the latest gtk+ (2.13.5).
In this version (see http://mail.gnome.org/archives/svn-commits-list/2008-June/msg04258.html\)
GtkFileSystemError and gtk_file_system_error_quark have been removed.
This causes ruby-gnome2 to fail to build from source, as well as several ruby-gnome2 applications to fail with an "undefined symbol: gtk_file_system_error_quark" error.
I've patched the Ubuntu version with the attached patch, which I'm submitting for your consideration.