Nicely done - this was a very interesting game! Gravity is essential indeed.
The concept of the universe ending if we don't pretend that gravity is still working didn't make much sense to me, but the mechanic was very cool.
I can just imagine an interactive story mission in something like GTA where the character goes "Wait, that's not how it happened!" whenever the player deviates too much from the ghost car - with lots of near-hits, slow-motion and explosions thrown in when the player does manage to follow the character's pre-recorded path.