I have met a lot of people in my short life who have NO idea why we (Italians) call tomato sauce, gravy. Here and now we will end this gravy controversy once and for all.
First thing you have to know if there is one great distinct difference (in our minds anyway) between SAUCE and GRAVY. The one major thing that makes sauce into gravy is the addition of meat. Turkey gravy, beef gravy and chicken gravy are all called GRAVY because they are made with that specific meat. Sauce is basically a tomato sauce cooked without the addition of meat as is the case with marinara sauce.
Traditional (in my family anyway) gravy was made with multiple meats. Mom used to include meatballs made with Pork, Beef and Veal mix (Sometimes called meatloaf mix in the supermarket). Then we had the Sausage which is my personal favorite. To keep it real authentic use both hot and sweet sausage and put a toothpick through the hot sausage so you can easily tell the difference. Now here comes the one that gets all the medigans, the braciole (pronounced brajole/brazole). Braciole is basically a thin flank steak seasoned and rolled. Mom used to make it rolled around hard boiled eggs (YUMMY!) or mozzarella, or sometimes just rolled without anything.
So there you have it.... Next time someone from out of town asks you why you call sauce "gravy" feel free to point them here.