Hi! 
Please consider getting a second goose. A single 'guard goose' for a flock isn't really a thing. Geese will alert to danger, and their size may deter some smaller predators, but that's about it. It's also very unkind to a goose to keep it alone. They are flock animals and need to have at least one friend of its own species. Just like you should never keep a single horse.
All that said, yes, a growing gosling should not eat layer pellets. If she doesn't like waterfowl pellets, you can feed her a chick starter feed that has a high niacin content, such as NatureWise Chick Starter. Put the layer pellets up somewhere that she can't reach them and offer her a feeder of the appropriate feed.
What breed is Lucy? How did she end up orphaned? Is there any chance you can find one of her siblings to keep with her?