Is Moral Nihilism a Morality?

[The Grand Nudger]

 And does a model of reality that doesn't include gods necessarily exclude them? 

No.   Most agnostics leave the possibility of gods open, but have no use for them in describing the functional aspects of our reality.  In that, gods could be very much like any other existent entity. 

Present in the universe, but not required, and not necessarily excluded from existence. For the majority of time that men have believed in gods...just as an amusing sidenote, this is how they conceived of them. Additional entities, immensely powerful, but not some component of the universe so inextricable that without their presence it could not exist or function. In classical pagan thought (which bled into early christianity) the gods themselves were derived beings. They were created, or born, or made, or a consequence of some other thing.

As an example, If I tell you that I don't think that zebras are required to make my engine start, that shouldn't be taken to mean that just because I've excluded zebras from the description of a functional engine that I have excluded them from reality. Similarly, if I told you that I don't believe in zebras on account of never have seen one outside of a fanciful story, I'm only telling you something about myself, not about zebras. From the point of view of agnosticism, nothing is being excluded from existence. I'm living in a zebra free worldview(even though zebras are very much real and existent), on the one hand because I don't think that the claim that zebras make my car start is true, and on the other hand because I think they're fairy tales. If we found a zebra, it would change my ideas about the entities that the world contains, but not my exclusion of zebras from how engines work. The functional aspects of my zebra agnosticism are unaffected by the presence or absence of zebras, neither claim, properly, refers to their existence in the first place.

It takes a gnostic commitment to necessarily exclude gods from existence (as a proposition). The difference can be expressed very simply with grade school level grammar. What is the subject noun and the action verb in each sentence?

I don't believe that gods exist.
Gods don't exist.

(believers tend to imagine that some sort of conceptual coup would take place, if only they could demonstrate their gods existence, as though an existent god would, by default, then also be required to explain this or that or make their additional claims true, or that we would fall to our knees on account of it's mere existence - but if they properly understood the worldviews atheists held they would realize that this isn't so, that all of their work was still ahead of them. Just as showing me a zebra wont change the fact that it has nothing to do with starting my car, nor would I immediately set out to build zebra shrines furnished with this picture. My worldview, even as a gnostic atheist, would be unchanged by the discovery of some existent god - I wouldn't expect an agnostic to have to deal with something meatier. )