How to disable/inactive some of the GtkMenu Items

Using:

gtk_widget_set_sensitive (menuitem,FALSE); // to gray-out
gtk_widget_set_sensitive (menuitem,TRUE); //to enable

You can use gtk_widget_set_sensitive(menuitem, true/false) to disable or enable the menu item widget.

Alternatively, if you used GtkUiManager and GtkAction to build the menu, use gtk_action_set_sensitive() instead.