This is getting well outside my area but my understanding is that if you were approaching the center of the earth gravity would gradually decrease until you have effectively no net pull at the core. This is because the mass above you is still attracting you too so at the core you’re pulled equally in all directions. Using the same principle you’d essentially be free-floating if you found yourself in a hollow “shell” planet, presumably because the pull from whichever area of the shell is close to you is offset by there being more shell pulling you away.