Vehicles can turn sideways and upside down and all that, but players can't. There is no point in making iron boots; you can't flip your blockhead to walk up a wall. If you change the gravity in a certain area (yes, an AREA; you can't change it for individual players as far as I know) then they'll just fall in that direction.
The gravity jeep uses a hacky script similar to Space Guy's spacetick stuff.