When dragging and dropping from current to available on the gang screen you will get this warning:
/usr/bin/sndcs_gtk:22: GtkWarning: You must override the default 'drag_data_received' handler on GtkTreeView when using models that don't support the GtkTreeDragDest interface and enabling drag-and-drop. The simplest way to do this is to connect to 'drag_data_received' and call g_signal_stop_emission_by_name() in your signal handler to prevent the default handler from running. Look at the source code for the default handler in gtktreeview.c to get an idea what your handler should do. (gtktreeview.c is in the GTK source code.) If you're using GTK from a language other than C, there may be a more natural way to override default handlers, e.g. via derivation.