Knowing virtually nothing of GR, and only hints of particle theory, this might be something of a naive question. If I've misunderstood somethings, I would gladly like to know why. Perhaps a more straightforward version of my question is this:
What evidence do we have that all the currently known elementary particles do in fact attract each other gravitationally, and in such a way that is related to their masses?
Hopefully this will help to illustrate things: putting aside other forces like EM, obviously some sort of attractive mechanism exists between matter. For reasons that I hope will be clear soon, I'll say that particles want to be with other like particles, and that with more of those particles present in some region of space, the more attractive that region is to other particles belonging to that same class (like a trendy celebrity vacation spot).
If our way of thinking about gravity is correct, then all particles attract all other particles so that the "class" is trivially the set of all elementary particles and there's no need for me to speak of partitioning particles into exclusively "attractive" classes.
But without testing to see that (non-EM/strong) attractions actually occur between all the different types of elementary particles (e.g. bosons, mesons), how would we know that gravitation is actually fundamental, and that our observations of mutual matter attraction haven't misled us into thinking the equivalence principle is true for all elementary particles? The only scale on which gravity has any appreciable effect is the macroscopic scale, where observable matter is dominated by protons and neutrons, since just about anything else decays too fast to collect. If our observations of gravity are dominated by only a fraction of the particle repertoire with particles of virtually the same mass, could we have mistakenly assumed that mass is a universal indication of gravity? Might cosmic rays say something about this?