It actually varies a lot from one Italian region to the next! It’s usually plated with some sort of butter sauce, sometimes with some woody sage, and other times with Parmesan. It’s even often served in a mac & cheese-like bake!  But it’s very commonly served here in the United States tossed in a tomato sauce. At the end of the day — like most pastas — gnocchi with sausage is extremely versatile.