The strength of the gravity between a proton and an (anti-)proton is 30+ orders of magnitude weaker than electromagnetism. The only reason we are able to notice gravity at all is because the electromagnetic forces between macroscopic amounts of matter (~10^23 protons) cancel out essentially perfectly, leaving only the gravitational force remaining. But the amounts of antimatter we can create and collect is extremely microscopic, so we can't use this canceling trick. Furthermore, we can't keep antimatter from annihilating for long enough to take long detailed measurements.