Yours has been a fairly frequent question in the dog forum, so you might want to ask or do a search there.
Our geese run five to seven pounds, as opposed to a giant's ten to twelve, but even my 38lb Brittany was quick to make the transition without special training. And when guest pups have balked at geese, we've just set them up at heel, as if for training retrieves, have them retrieve a familiar bumper or duck a time or two to prime the pump, then tossed the goose. Hasn't failed, yet.
If you think I'm wrong, you might be right.