Even the heaviest of science-fiction postulated elements, (if Z=250 you might have over 800 nucleons), are still extremely, extremely small, which are the ranges over which both the electromagnetic force and the strong force have appreciable effect. Compared to those two forces on that distance scale, gravity is a wimp. In order to have a significant contribution from gravity, you would have to have something the size of a neutron star, because gravity just isn’t strong enough to hold anything smaller together. In other words, you can’t have a piece of a neutron star, because that piece’s own gravity would not be sufficient to hold it together.