salt and pepper
If you want a lighter colored gravy like you might get at a diner with chicken fried steak then use milk and use lots of pepper. If you want a darker gravy with something like mashed potatoes or a roast then use stock or broth instead of milk. You can also mix them (I like a 50-50 mix on toast).
If you already cooked some kind of meat then using the pan drippings to replace some of the butter is really really good.
This works with dairy free options as well.