You’re overthinking this way too much. Gravity is a force that exists, it’s undeniable. Plato lived thousands of years before it was even discovered, he’s not relevant to this
Rather, I’m just using words your unfamiliar with and topics which you perhaps haven’t yet had a chance to think critically on. The belief that abstractions such as “the law of gravity” are true apart from any other reality is commonly known as Platonic realism. Likewise, the idea that if you have one of something, and then have two, you’ve doubled it. Is that simply true, apart from any other reality? 2+2=4? Snow is white (when by white we mean that it reflects a certain band of EM radiation which, when detected by the human eye, is what we call white)? If you hold that these are true and are true apart from any other reality, you are a realist, and more specifically a Platonic Realist of some sort.
Platonic Realism is simply the term used to describe what you’re saying. The fact that you say the law of gravity is itself the cause of something is a further hint.
I could give you an out, of course. While it’s true that many mathematicians hold to some type of Platonism, certainly not all scientists do. And many scientists no longer take the Platonic view of “laws of nature” being abstract but real and true entities in themselves, but simply man made descriptions for the way things interact with each other based on their intrinsic nature. That is, the laws of nature do not do any acting or causing. They’re just man made conveniences.
Of course, however you answer I still have to point out that you previously stated “everything has a cause,” and when pushed on that you firmly committed to it. So if you’re a Platonist, I need to then ask “What caused the law of gravity?” and any other abstract truth, for indeed if these are true and therefore real in themselves, and if everything has a cause, so must these. But if you deny Platonism and simply take the laws of nature as man-made descriptions of the way things behave by their own intrinsic principles, the statement “the law of gravity caused the big bang” can no longer be true, in which case I must ask "what caused the big bang? since the law of gravity will have been ruled out. Or you could also retract the statement “everything has a cause.”