I've only heard of using them in large fields of one crop that geese really don't care for. Potatoes were mentioned for sure, as was corn when it gets larger. Ideally you would feed them piles of the weeds found in your garden as goslings, and then they would be all primed to eat as soon as they were old enough to go outside. Then you fence them into the patch - it has to be big to supply all their food. The key is to put them into the field when the crop was large but the weeds were small, so they would still eat them. (I can see them not liking potatoes, but corn seems a lot like goose food to me. I can see all the leaves clipped short)
I gave up on the idea as soon as I saw the geese foraging. They don't like broad leaved weeds, which then go to seed creating more weeds. We have a lot of thistles, and they don't eat those. They are great for grasses and clovers, but will eat all their favorite bits out of the pasture, and prefer grasses that are on the shorter side. They do a great job if you also mow, but otherwise the pastures end up being kind of odd looking, with clumps of weeds here and there, and very short grass. Plus, they have a random habit of just nibbling on things. I don't trust them with garden plants. They ate the new growth off of my fir trees, which they are supposed to not like, and some tops off of the raspberries. I shudder to think of them loose in the main garden . . . Plus, it wouldn't exactly be pleasant to work in the garden with goose poo all over the ground . . .
Now, I still have the geese -- love them and their personalities. They do eat a LOT of grass and they save a bit on mowing in our fallow pastures -- we just have to mow every now and then to keep the thistles and weeds down. The geese take care of the grasses, and fertilize next years fields at the same time. My American Buff are calm, pleasant geese with great personalities, and they are very active in foraging around for grass . . .
As far as teaching the ducks -- ducks are born knowing how to forage. They are more successful in the gardens because they don't eat as many plants, and they so love bugs and slugs. They will eat some garden plants though, so you'd have to supervise until you know what they like and don't like.
