Everytime you hatch a Pokemon it will be in it's NFE (Not Fully Evolved) form. So you will have a Ghastly.
Although unless you breed your Gengar with a Female Gengar or a ditto you won't get a ghastly. The Pokemon species depends on the female, male if you have a ditto.
Although the base point is the same unfortunately, it will still be a Ghastly, not a Gengar.