Quantum fluctuation question ...?

>If I am getting this right, nothingness is unstable.

I don't really think this is a correct statement. First off, I don't know what you mean by nothingness. You could mean the vacuum of space-time, or you could more literally mean absolute nothingness, not even space or time. In either case, it does not have any properties which allow it to be stable or unstable. There is the Heisenberg principle in quantum mechanics which allows for the spontaneous creation "virtual" particles out of nothingness (note the virtual particles don't actually exist so there's still nothing there). Possibly that is what you are referring to when you say nothingness is unstable.

>When we apply gravity, we have the potential for positive and negative energy fields with a net value of zero which allows something to come from nothing.

Again, this sentence makes no sense. The notion of negative energy is not real. You can finagle a system to have negative gravity if you want, but that does not make it an inherent property of the system. In a similar fashion, we as scientists have created the idea of potential energy, but in reality, potential energy does not exist. It is merely a mathematical tool to simplify things. Likewise, negative energy does not exist. We can construct a theory around such an idea to help describe something, but that doesn't mean negative energy exists. I also don't follow how having a net energy of zero means something can come from nothing. Such a thing is impossible (within the physics of our current universe).

>But where is the gravity coming from?

How should I know? You're the one who said we should apply a gravitational field. You can't say something like, I'm going to drink a cup of water, but where did the cup of water come from?

>I thought gravity was the bi-product of mass warping spacetime.

This is one approach to gravity, yes. General Relativity treats gravity not as a force, but rather the warping of space time with a stress-energy tensor that derives from bodies with mass or energy.

>Wouldn't that mean that we would already need to have both spacetime and mass in order to get the gravity in the first place?

Yes, you would need spacetime to be warped and the mass to warp it if you wanted gravity to exist. However I don't see how this plays into the rest of your question at all.

EDIT: Okay, those clarifications clear somethings up. First off, and this is very very important, no theory in science ever says anything about what created the universe or how. The idea of quantum fluctuations relating to the origin of the universe is merely an idea to explain how the universe started it's initial expansion. The Big Bang theory posits that initial there was a "primeval atom", basically the entire universe condensed to an extremely small area. It does NOT state how the universe came to be or what put it in that state or what it was like before that state. The Big Bang theory even has trouble saying what caused the initial expansion, but one idea is that quantum fluctuations caused it. But note that the fluctuations happened in the already existing universe; a universe which already had space, time, and energy. Your more ethereal question of where did it all come from to begin with is currently outside the realm of science. It cannot be answered with what we know today. I know this may be difficult, but so far you will just have to accept that in the beginning the universe started as a certain state and try not to ask how it got like that or where it came from. Unless we can possibly find observable evidence to indicate an answer, we are completely blind.

@Bulldog Drummond - Yes it is true that vacuums can produce virtual particles which have measurable effects. But it is important to note, and what I was attempting to point out, that you never have something coming out of nothing. In all cases, when the virtual particles interact with normal matter and don't annihilate immediately, they must "steal" energy from normal matter in order to come into existence. In this way they are merely taking from what already exists and so they aren't coming from nothing. As for whether vacuums are stable or unstable, a vacuum could be unstable, but a true vacuum, as most of the universe is, is in it's lowest possible state and thus is stable. You could have false vacuums, which are unstable, but they are unstable because they have the potential to "degrade" into a true vacuum, or zero-energy vacuum. Being stable or unstable is not a matter or how or why virtual particles exist.

EDIT 2: I'm running out of room to respond, but yes, quantum fluctuations have to happen in our universe.