So, wait, gravity: is it still pointing towards the core? So the inhabitants will perceive the Higher World being beneath the Lower World?

If you were talking about variable gravity, it's actually not that difficult to think about if you do it properly for the players, but it requires a lot more effort in terms of the DM. For example, consider a cylinder, with either the gravity constantly pulling them away from the center or towards the center. Both of these can be demonstrated with maps, but your measurements will probably be difficult to fit into a 5ft by 5ft grid.

From experience, my players had a lot of fun with this mechanic when I included it (gravity constantly pulling towards the outside of cylinder and shooting across the middle of the cylinder). It's complex, and you'll probably want to have some visual aids to clarify what you mean, but not difficult to grasp.