Spend much time with historic texts, and you get the feeling Inigo Montoya is standing over your shoulder. “This word you keep saying, I do not think it means what you think it means.”
In 19th century and earlier cooking, the term “gravy” is used to refer to the drippings and juices from cooked meat, or the liquid in which it is cooked. The finished gravies of modern cooking were called “sauces” in that time. Thus, a typical brown gravy of today would be called a “Brown Sauce”, and cream gravies of today would be “Cream Sauce”. Additionally, anything called “sauce” today would also have been called sauce then. The simple definition would be, if you would put it on the dining table, it is sauce, not gravy.
But, the definition of “sauce” can also refer to the vegetables served with the meal[1]:
Long Sauce. Beets, carrots, and parsnips are long sauce. Potatoes, turnips, onions, pumpkins, &c, are short sauce. See Sauce
Sauce. 1. Culinary vegetables and roots eaten with flesh. — Webster.
So when you see a word that seems out of place, consider the context. It may not have meant the same thing then as it does now.