Again? Oh, for crying out loud! :lol:
Well, I don't think the "not thought possible" bit is accurate: One of the things that LIGO has explicitly been looking for is events involving a body in the mass gap. This isn't something that they weren't looking for because it wasn't thought possible and therefore came as a huge surprise. Basically, we don't have the equation of state for nuclear matter very tightly constrained, as we only have atomic nuclei and the results of particle collisions to work with in terrestrial laboratories, and there isn't a convenient neutron star in the solar system for us to examine from up close. So we don't know where the boundary between neutron stars and black holes lies, and the set of known objects doesn't contain enough objects close to the boundary to constrain its location well. Gravitational wave observations let us search a larger area of the universe, plus the fact that they involve mergers potentially allows us to probe the interior dynamics of neutron stars by seeing how, e.g, tidal deformation of the neutron star affects the GW spectrum, and/or, for closer events, by analyzing the electromagnetic emissions from the debris.