say, I want to make a both sided flat world - plane with things on both sides, having gravity pull them towards it. Or a spherical world having its own gravitational pull. Are any of these possible?
Yes, add a force field of type Force. Put it to the center in order to create a spherical world having its own gravitational pull. Use shape "Plane" to have a both sided flat world. As the manual says:
Shape The field is either a Point, with omni-directional influence, or a Plane, constant in the XY-plane, changes only in Z direction.
Strength The strength of the field effect. This can be positive or negative to change the direction that the force operates in. A force field’s strength is scaled with the force object’s scale, allowing you to scale up and down scene, keeping the same effects.