To keep them legally you have to purchase from a seller who has the registration and paperwork that is given to you when you purchase them. There are not many who sell goslings, and possibly nobody who sells hatching eggs.
The geese that are yours, have to be marked in some way. One way is by removing one back toe, and I think the wings have to be clipped.
FYI, since they are protected it is illegal to collect eggs or take a gosling from the wild.
If you have a large pond, keep it mowed around it, and brush cleared away, and possibly some wild ones will come, that can hang around.