code: func (ps *PrintSettings) GetLength(key string, unit Unit) float64 {
  	cstr := C.CString(key)
  	defer C.free(unsafe.Pointer(cstr))
  	c := C.gtk_print_settings_get_length(ps.native(), (*C.gchar)(cstr), C.GtkUnit(unit))
  	return float64(c)
  }
docs: // GetLength() is a wrapper around gtk_print_settings_get_length().