Let's look at the weak interaction first, it has a very similar situation: A top quark is so heavy that it can decay to a bottom quark plus a W boson. The W boson then decays to other particles. How can a neutron decay via the weak interaction? It's much lighter than a W boson, it cannot decay to it. It still couples to the associated field, however, and that couples to the decay products of a neutron. You never produce a real W boson in that decay but it allows a neutron to decay to proton+electron+antineutrino. Mathematically we can calculate the probability of this process using virtual particles. They are not real (hence the name), but they have some similarities to the real particles.

Back to gravity: If you shoot two protons at each other with an absurdly high energy then you can create a black hole. The black hole will then decay to a variety of particles, could involve protons but it doesn't have to - black holes don't differentiate between matter and antimatter. Random protons in a cold Earth don't have that energy, but they still interact via gravity, so just like for the W boson case there should be a decay process via virtual black holes. We can't calculate what proton lifetime that will produce (besides "absurdly long") and of course we cannot confirm something experimentally that we don't expect to happen even a single time over the next quadrillion years - but the process should be possible.