I have had asphalt millings placed on a gravel based driveway. It has been compacted. Is sealing it a good idea?

No, if you plan on getting the topping or finish coating later. That base layer will need to be cleaned before the topping, so the topping's heat can re-activate and bond with the base's tar. If what you have is all you're going to have (bad idea and a waste of money), then yes, go ahead and seal it. What you have holds water and if you have freezing temperatures it won't last long and needs whatever help you can give it. You should re-seal it every year regardless.