I don't know anything about organic garden ducks, but organic ducks themselves haven't been treated with any chemicals. They aren't fed any hormones or antibiotics and are raised as free range so the poultry doesn't undergo unnecessary stress. I'm guessing organic garden ducks are organic ducks raised in a garden-like setting...?
An organic duck is a duck, raised in captivity, like on an orchard, just to be eaten. Some people raise ducks, just to eat them, like chickens. My friend actually has some 'organic ducks' that he raises and eats.