It is reasonable to say that the idea of gravity, or its coinage, does not predate Sir Isacc Newton. But in ZMM Pirsig isn't talking about these things. He's talking about the law of gravity and gravitation itself. He has some very unusual ideas about these. For those who have read this passage and found it reasonable, I suggest that you've applied your own static filter to "fix" the MOQ wherever it seems nutty, silly, and daffy. Because here it is nutty, silly, and daffy. Here's the full text of the gravity passage, with my comments interspersed.
...it seems completely natural to presume that gravitation and the law of gravitation existed before Isaac Newton. It would sound nutty to think that until the seventeenth century there was no gravity.
- Pirsig, p. 40 (ZMM, 25th Anniversary Edition)
Pirsig is careful about what he means here. He clearly states that gravity and the law of gravity are two different things. The names or phrases "gravity", "law of gravitation", and "idea of gravity" are intellectual shorthand and is a third type of thing, but he doesn't mean those here. If that's all he meant, it wouldn't "sound nutty".

So we have:
1. the inverse square law which describes the behavior of masses in the presence of a gravitational field.
2. the actual force itself that obeys the law in 1.
3. the terms "gravity", "law of gravitation", and the "idea of gravity": intellectual shorthand for 2., 1. and the combination, respectively.

For each of these you can ask if they existed before Newton. Pirsig continues...

What I'm driving at is the [ridiculous] notion that before the beginning of the earth, before the sun and the stars were formed, before the primal generation of everything, the law of gravity existed. Sitting there, having no mass of its own, no energy of its own, not in anyone's mind because there wasn't anyone, not in space because there was no space either, not anywhere-this law of gravity still existed? If that law of gravity existed, I honestly don't know what a thing has to do to be nonexistent. It seems to me that law of gravity has passed every test of nonexistence there is. You cannot think of a single attribute of nonexistence that the law of gravity didn't have. And yet it is still 'common sense' to believe that it existed.

Well, I predict that if you think about it long enough you will find yourself going round and round and round and round until you finally reach only one possible, rational, intelligent conclusion. The law of gravity and gravity itself did not exist before Isaac Newton. No other conclusion makes sense. And what that means is that the law of gravity exists nowhere except in people's heads! It's a ghost! We are all of us very ignorant and conceited about running down other people's ghosts but just as ignorant and barbaric and superstitious about our own. [stress his]
Pirsig clearly says that the law of gravity and gravity itself did not exist before Isaac Newton. He's not talking about mere word coinage here. Not only has he demoted the law of gravity to a human concept, but gravity itself. He doesn't bother to explain what must have held the sun, solar system, and galaxies together before gravity came into being, circa 1600 AD.

However, I have an idea what he thinks held the cosmos together before 1600, and it was the Ptolemeic world view, a static intellectual pattern of the universe created by Aristotle having 8 concentric spheres each holding a moon or planet, with the outermost having many small points of light affixed to it, and rotated by a God that stood outside them. The Earth was at its center and it got more divine the farther out you went. Apples fell to earth because apples are composed of earth and they seek their rightful place, etc.

And make no mistake this system was not just a "wrong concept", as we think of it in retrospect, but it was the static reality before 1600. Running with Pirsig's argument we find that if Aristotle had a means to investigate the heavens close up, he would have found those spheres, touched those affixed stars. And why not, if reality is what you invent?

I think Pirsig would say that stars, galaxies, and supernova simply did not exist back then. These "concepts" were invented after that and static reality shifted toward these as they became accepted. In short, according to Pirsig, no static reality exists beyond what we invent and subsequently brainwash ourselves into believing, and indeed, one of these beliefs is gravity, another is a rock we hold in our hand, and another is objective reality itself.

When we speak of an external world guided by evolution it's normal to assume that it is really there, is independent of us and is the cause of us. The MOQ goes along with this assumption because experience has shown it to be an extremely high quality belief for our time. But unlike materialist metaphysics, the MOQ does not forget that it is still just a belief - quite different from beliefs in the past, from beliefs of other present cultures, and possibly from beliefs we will all have in the future. What will decide which belief prevails is, of course, its quality.
- Pirsig, The Role of Evolution, Time and Order in Pirsig's "Metaphysics of Quality", by McWatt, Priezkalns
In this quote it appears Pirsig does an about face and flatly admits that objective reality exists. (No wonder that even the long-timers argue to this day about what he truly means.) But let's not be too hasty. He will only go so far as to say that it is an "extremely high quality belief of our time", but like any other belief, it's future prevalence is unclear. So while he can understand why other people would believe in objective reality, and indeed the MOQ levels suggest an objective reality, his personal position remains unclear.

While Pirsig is quite right in pointing out that world-views change and that different cultures have different beliefs, he is dismissing the obvious. Some cultures are more technologically advanced and know more than others. If your culture does not have telescopes in big observatories, and is isolated from cultures that do, then your culture is not going to know what the stars are made of or that other galaxies exist. And the world-view in Aristotle's time was indeed invented. It was guess-work, conjecture in an ignorant time. They never bothered to test their ideas. How can he compare this to what we know and have accomplished now and make similar conclusions? NASA just landed a space probe on an asteroid only 8 miles long and millions of miles away. What are we to say about this? We have a pretty good grasp of our beliefs? Sure, all that science claims is still a belief, but what's it gonna take to turn this into "I believe"?

We are very confident that light from distant galaxies takes thousands of light years to reach us, and also that gravity, Newton's gravity, along with velocity, is responsible for the spiral shape of these galaxies. So we know that gravity is at least thousands of years old (indeed much older), and not merely 400 years old as Pirsig claims.

Pirsig is saying that evolution of static reality is correct and the ability humans and cultures have to create reality is also correct and he bundles these two notions under the MOQ. But I think I've shown that the two notions are not compatible and cause internal contradictions within the MOQ. You can have one or the other, but not both.

Glenn's Postmortem MOQ Page