Some have one or the other some have both. I personally prefer to not have an undergravel filter.
The purpose of the filter is not to just filter the water. Of course it does take the main debris out of the water but it's a place for bacteria to grow to cycle your tank and reduce ammonia and nitrite which are deadly to fish.
Main thing with gravel is to clean it with a gravel vac or to swish it around to get the poo and uneaten food.
" To reach manhood, we are all cast into the fire and purified by pain "
Water filter? If you mean like a hang-on-back aquarium filter, that is all you need if you have the right size for your tank/fish load. IF you mean like Reverse osmosis or a Brita pitcher that filters the tap water, it won't cut it.