If I set a stop-loss order at a certain $$ amount, and overnight if it gaps down (eg what happen to Keppel), ie bypasses the pre-set amount, does it still work?

No, your Stop Loss Order will not be filled. It will turn into a Market Order and your position will be closed at a lower price leading to a greater loss. Unless you use a Guaranteed Stop Loss Order (GSLO), which incurs a certain amount of premium.