From what I understand. The term gravy - meaning spaghetti sauce comes from early 20th, late 19th century Italian immigrant woman. Who brought their recipes over from the homeland. Another old proper name for spaghetti sauce is Ragu.
My mom who grew up in Fort Hamilton and Bay Ridge Brooklyn had many a friend in her yout who were very Italian(if you know what I mean), and she was taught at an early age that the proper way of asking for extra sauce was "Can I have some extra gravy on my pasta".
Chris