You may want to check $nick != $me so that you would be able to override your own script if need be.
on @*:MODE:#: {
if (G isincs $chan(#).mode && $nick != $me) mode # -G
}
Also if a server changes the mode to +G (possible scenario) you may need the same script running from an ON SERVERMODE event (I'm not sure if servers trigger ON MODE in addition to ON SERVERMODE).