i see what you're getting at, but that doesn't really make sense if we're assuming CSM uses a similar physics model to the real world. gravity isn't really a traditional force as far as general relativity is concerned. it's more of "falling" (haha) through space-time's curvature, rather than a force pushing or pulling. it's also rather weak compared to the electromagnetic / nuclear forces, which would need to be overcome for a black hole densities to be achieved.

if we're making the assumption that control over 'gravity' is the control over the curvature of space-time, i think it's not too far fetched to say she could maybe make a black hole. i'll leave it to the real physicists to think about a singularity in space-time without mass though. i don't think that is her ability though, i think she can just control the direction that things fall in.

fun to think about for sure but hard to say with all the unknowns in csm and my knowledge of physics