Gaus's law is easier to visualize for electric charge than for gravity, but it also works for gravity. If i have a collection of charged particles, protons, let's say, and there's no electron, so it's just protons, then what gaus's law tells me is that, if i look at the electric field on the boundary of that region. So we figure out a lot about the distribution of charges inside, just by thinking, just by measuring the electric fields on the boundary. We can't learn everything about the charge distribution inside because if you have a charge distribution that itself is perfectly spherical, then the size of that sphere is completely irrelevant.