I fed mine chick starter and added Nutritional yeast to it since chick starter doesn't have enough niacin in it that water fowl need. 1 Tablespoon per cup of feed.
When are your gosling arriving?
I bet that was a surprise. Congrats!! Sometimes when they first begin to lay they can lay soft shell but I keep oyster shell out for all my egg layers or you can feed layer and keep the OS out too.
@Madonna m you might get a kick out of this too. https://www.backyardchickens.com/t/911816/how-to-tell-a-goose-from-a-gander-in-case-anyone-is-interested