Will empirical evidence arise through experiments definitively determining if gravity is fundamentally quantum? Or will evidence prove that gravity cannot be quantized into Graviton-like particles? Multiple experiments have been proposed, and some are currently underway which may answer this fundamental question. Market resolves to YES if definitive proof arises as to the nature of gravity by the end of 2030, and otherwise resolves to NO.
A related market:
You’re going to need to specify the threshold more clearly. There is a spectrum of proposed tests that are progressively more challenging to perform but which make fewer assumptions (and hence give stronger evidence that gravity is quantum). It wouldn’t surprise me if the easiest ones are achieved by 2030 but the harder ones not until many years later. (Indeed, many theorists already consider our current data to be overwhelming evidence...)