If you post pics, more than likely we can assist in saving the plants rather than just writing them off. Overwatering is NOT going to help with the Fungus gnats. I often have all out wars with the little bitches. A good layer of sand or perlite on top of the soil along with letting the soil dry in between waterings should kill the gnats off. Throw a couple fly strips up to catch the flying ones. The sand or perlite will keep mature gnats from getting to the soil and laying eggs. Letting the soil dry up in combination with the sand or perlite will kill off those nasty, root eating larvae.