[uewnovfl] Gravity on a cube world

Consider playing Minecraft not on a plane but on a very large cube.  How would gravity work?  Where would "down" change direction?

Drop down to two dimensions for simplicity.  Square planet.  Divide the universe into 4 trapezoidal regions with boundaries extending diagonally from the corners of the planet.  Gravity points orthogonally down in each region.  On the exact boundary line, gravity points diagonally.  Objects in deep space falling under the influence of gravity will likely hit a diagonal line then crash onto a corner of the planet.  This scheme remains consistent underground.