If the bottle is falling, then its gravitational potential energy is also falling. That's because gravitational potential energy is (mass)·(gravity)·(HEIGHT) .
So the best I can tell you is that at any instant of time, the bottle's GPE is
(2 kg) · (9.81 m/s²) · (HEIGHT above ground at that instant) .
That's going to be (19.62 x height above ground in meters), and the unit, of course, will be 'Joules'.
