You have to remember that General Relativity is wrong. We don't know exactly how it's wrong, but it's wrong. In light of that fact, it's not really a proper answer to say that "General Relativity says it isn't so it isn't." It is true that GR says gravity isn't a force, but the implication doesn't follow. We don't know whether a Grand Unified Theory would have gravity as a force or not. Given that a great deal of the answer to "Is gravity a force?" depends heavily on the definition of force being used, we don't even know what the final definition will be in light of a GUT. There may or may not be a meaningful distinction between gravity and other forces in the real universe.
In QM, gravity isn't a force because gravity doesn't exist. Clearly this is wrong as well. Saying "GR says gravity isn't a force" is effectively the same thing as saying "Gravity doesn't exist because QM says it doesn't." This is exactly where both theories break, so we can't use them to answer this question.
I'm not seeing how these distinctions are helpful. Maybe there's a miscommunication?
When we say "gravity isn't a force", it's shorthand for "our best understanding of gravity, with the widest explanatory power that most accurately matches experimental results, says that gravity isn't a force." But that's an exhausting, verbose way to talk. Knowing that GR is incomplete doesn't change that.
No falsifiable theory is 100% guaranteed Truth. Not GR, QM, evolution, or any widely accepted future Grand Unified Theory. Science is one big exercise in affirming the consequent, verifying contrapositives, and finding surprises. Maybe all mental models are wrong. Maybe we're brains in vats. Aaaah!...so what?
Evolution naturally developed eyeballs in mammals. My niece just turned 5. The atoms composing me won't rearrange into a facsimile of Abraham Lincoln tomorrow. I'm not going to add "to our best understanding, given the evidence, granting my incomplete knowledge, etc" about these facts in service of reality obviously being unknowable. It's pointless.
Gravity isn't a force. That's a perfectly "proper answer" -- no need to pontificate about philosophical relativism.
This isn't philosophical relativism. I don't deal in that.
This is precisely where GR doesn't work, so using it to declare with confidence what gravity "is" is as silly as using QM to do it. They don't work on this matter. General Relativity is definitely not a complete description of how the universe works. It's a good approximation, even a very good one.. but no more than that.
We know this. It is well understood. This is not some sort of whacky Time Cube position. The whacky Time Cube position is really the one that says GR is correct so we can declare with confidence that gravity isn't a force.
In the end I don't expect gravity to be different from electromagnetism and the other forces. There's been plenty of efforts to build GUTs from nothing but geometry in which case nothing is a "force". Efforts from the QM side tend to incorporate "gravitons" as the force mediation particle just like the other forces have mediation particles. It seems very likely to me that GR's "gravity is specially not a force" will end up being an error, in that either all the other forces turn out not to be forces either, in which case we'll change the meaning, or it will be a force like every other. I don't think there's a lot of reason to expect this to be anything other than an artifact of GR being an approximation.
We don't have a single test that shows that GR is wrong in any way and we have many tests that show that it is exactly right to any arbitrary precission we can dream of achieving.
GUT does not unify gravitation; that would be a Theory of Everything (TOE). GUTs only seek to unify the strong and electroweak forces. TOEs are even more speculative than GUTs. AFAIK there are no purported GUTs which do not use gauge groups, so I don't understand your point about "force".
> In QM ... gravity doesn't exist
The Standard Model of Particle Physics, a particular relativistic quantum field theory, is Lorentz-covariant by design (and with good reason; Lorentz symmetry is a highly-tested feature of our universe). General Relativity as a physical theory of our universe is defined on a Lorentzian spacetime. Where the radius of curvature is larger than the Compton wavelengths involved, adapting the flat-spacetime Standard Model to curved spacetime is straightforward textbook stuff. (Wald 1995 ISBN 0-226-87025-1; Birrell & Davies 1982 ISBN 0-521-23385-2 and several others).
For smaller radiuses of curvature, of if the semiclassical approach is unsuitable because of nontrivial superpositions, one can take a second-quantization approach ("CQG", very definitely a QM theory in which gravity "exists" as a quantum field with potentials and dynamics, and which admits the correspondence principle) or <https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd...> both of which match General Relativity as effective theories (the characteristic energy cutoffs are usefully large and/or the characteristic cut-off lengths are transplanckian) are verified to high precision in manifestly relativistic systems (e.g. triple PSR J0337+1715). Are these "wrong"? Prove it. You'll likely need to look into a region of strong gravity, and for that you have to deal with a cosmic censorship conjecture.
> General Relativity is wrong.
Prove it?
I'm with Clifford Will <https://en.wikipedia.org/wiki/Clifford_Martin_Will> on this; penultimate paragraph of his 2020 book with Nicolas Yunes ISBN 9870198842125, <https://global.oup.com/academic/product/is-einstein-still-ri...>: "It would not surprise me if the solution to the universal acceleration turned out to be simply Einstein's original cosmological constant, a constant of nature like Planck's constant or Newton's constant of gravitation ... And it would not surprise me if general relativity turned out to be absolutely correct according to any future experiment accessible to humankind." The authors' following paragraph is a challenge to you which is not met by your bold assertion at the top of your comment.
General Relativity may be wrong, but that is not proven. Will would be happy to be surprised by it being proven wrong experimentally. (A huge amount of his research has been aimed at what exactly it means for General Relativity to be wrong).
What I would agree with is that we do not know at present how to avoid divergences of the curvature scalars in the limit of strong gravity, and that this means we can't always fully solve a whole spacetime from initial values. But we also don't have any idea how to test whether that actually happens in our universe (that such divergences occur in stellar remnants or at the centres of galaxies is untested hypothesis) so GR is a perfectly sound, mathematically complete effective field theory at the very least.
And moreover the answer may come from new physics in the matter sector, rather than new gravitational physics. What does the Standard Model say about particle energies above YeV? Discovering more about the mechanisms that generate the stress-energy tensor is not the same task as changing the theory of General Relativity.
Meanwhile, there are hundreds of pages in Will & Yunes's excellent book which explain to lay readers the match between GR and observation and experiment, and a wide body of specialist literature on a century's worth of experimental and observational tests of General Relativity, every single one of which has supported the theory or was inconclusive.
In QM, gravity isn't a force because gravity doesn't exist. Clearly this is wrong as well. Saying "GR says gravity isn't a force" is effectively the same thing as saying "Gravity doesn't exist because QM says it doesn't." This is exactly where both theories break, so we can't use them to answer this question.
See also https://www.youtube.com/watch?v=EmrZ3ZaXmS4 .