If you look at the quantum energy levels, the proton and the antineutron, being distinct fermions, can occupy the same energy level (in this case the 1s orbital) and so would be literally in the same place as opposed to far away.

Secondly, the nuclear interaction inside nuclei essentially consists of nucleons swapping quarks with eachother (and creating virtual antiquarks so overall a meson is the exchange particle). So if the proton and the antineutron were bound then an up quark would move from the proton to the antineutron but interact with the antiup quark there and annihilate.

If you are talking about surface gravity, and wanted to calculate the surface gravity of a planet in terms of Earth's gravity (g=(9.81ms^-2), then you couldn't just use the masses of the planets, you'd also need the radii. For example if you wanted to calculate the surface gravity of Mars, 0.38g.

The surface gravity of a planet is calculated as (4/3)*pi*G*density*radius, where G is the universal gravitational constant. If you know the density (mass/volume) and radius of Earth and the planet you're standing on, then the ratio of your planet's surface gravity to Earth's is the ratio of the densities multiplied by the ratio of the radii

or: g_planet/g_earth = (p_planet/p_earth) * (r_planet/r_earth)

More generally, the classical gravitational field at a point due to a distribution of masses is

Div(g) = -4*pi*G*density