When the gong is rung, it will display different icons all over it. For example, it just granted wisdom, and it had sparkles all over when it was rung. When it takes health, it has green/gray chemical vapors all over. These icons can help determine what the gong is granting, be it a bad thing or good.

So I think the answer to whether the luck is good or bad may lie in what icons it displays when it's rung. Does anyone know what icons it shows, and does it always show the same icons when it grants luck?