GtkBuilder and its schema doesn't have anything to do with scripting in GNU applications or Guile.
Vala is also not relevant.
What the GNU's statement above is about is an embedded scripting language for GUI application extensibility (to write things like editor plugins in, to provide scripting for a GUI app like GIMP, etc.).
GtkBuilder is a UI builder for Gtk apps with a XML-based serialization format. It solves a totally different problem.
Vala was an attempt to replace C and have a "modern-ish" language with first class GObject support. Again, solving a totally different problem.
