The fact is that nobody knows. We know that gravitational waves exist, but there is no real experimental test which could tell us that these waves have quanta the same way electromagnetism has photons. It seems overwhelmingly likely though, for the simple fact that matter causes gravitation, but matter is fundamentally made of particles subject to quantum uncertainty, and it is difficult to imagine how this quantum uncertainty would not get transferred to the gravity. For example, if an electron is in a quantum superposition of being in place A and in place B, what is its gravity? Half here half there? Does the superposition vanish when you measure the gravity? It should! So, in order not to mess up the logic of quantum mechanics, the gravitational force needs to play by the same quantum rules, and that would usually entail the existence gravitons.
The fool hath said in his heart, There is a God. They are corrupt, they have done abominable works, there is none that doeth good.
Psalm 14, KJV revised edition
