I am heavily recommending this example (Which is bit old but the right way to deal with context menu).
lable.sinkEvents(Event.ONCONTEXTMENU);
lable.addHandler(
new ContextMenuHandler() {
@Override
public void onContextMenu(ContextMenuEvent event) {
event.preventDefault();
event.stopPropagation();
popupMenu.setPopupPosition( //custom menu here
event.getNativeEvent().getClientX(),
event.getNativeEvent().getClientY());
popupMenu.show();
}
}, ContextMenuEvent.getType())