I would have to agree with KAhunter if possible try corn there is no duck that i have come across that will not eat corn. also according to stanford university this is the typical gadwall diet. Grass, aquatic plants, nuts, grain; aquatic invertebrates, small vertebrates. Dives for food more than most dabblers. In winter, mostly aquatic veg, algae. hope this helps some their info is somewhat vague but if possible try the corn idea thats what i'm doing for woodys teal and mallards at my place.
You know your hunting in South Carolina when a bunch of ducks fly in and you shot both of them!!