I have to agree with Luke on this point, Jonathan. Do you really not see the distinction between the situation of an observable/detectable photon, or electromagnetic particle/wave, and an unobservable, undetectable so-called 'graviton'?
You can observe the existence of gravity in the same way you can observe the existence of electromagnetic photons--by measuring their effect on/interactions with matter. You can measure photons with a photodector, or an antenna, depending on the frequency of the photon. These devices convert photons to an electrical signal that we can measure, amplify, digitize, or synthesize into a sound or image.
You can similarly measure the acceleration effect of gravity by using a piezo sensor or laser interferometry to create an electrical signal to measure the deformation of a mass with a known elasticity. You're using a defferent sensor to create the electrical signal, but the concept is exactly the same.
Also, note that there are situations where electromagnetism can exert a force without photons being involved, such as the attraction between the plates of a charged capacitor, and the attraction or repulsion between two adjacent magnets fastened to a rigid object. In those instances, it is unknown whether there is a particle that carries the force from one object to another, but observing that the force exists is easy. All you have to do is place a pair of 2x2x1" NdFeB magnets on opposite sides of your thumb, and you will quickly observe a variety of phenomena that will vigorously confirm the existence of a force between the magnets. And if you receive immediate and competent medical attention, you may even be able to continue to use your thumb!
You are correct to say we don't know for sure if there is any such thing as a "graviton" particle that carries the force of gravity from one mass to another. It is also correct to say we don't know if there are particles that carry forces from one electrically charged capacitor plate to another, or from one stationary magnet to another. That doesn't make the existence of any of those force phenomena "unobservable".
We can know something exists without understanding every aspect and detail of its existence. Otherwise it would be impossible to know anything without knowing everything.