They key part here that is not easy to set up in an experiment is "massive object in superposition". Superposition is generally something that we observe in mass-less objects (bosons) like photons on a macroscopic level. The spin state of an electron or any other massive objects (fermions) isn't going to effect it's mass distribution observably on a macroscopic level as far as I understand it...these things usually collapse upon interaction with other particles.

The curious thing in my opinion is how this will effect quantum computing. If gravity causes decoherence, then it is likely difficult to maintain coherence in large qubit devices that aren't in microgravity.