The work done is the change in gravitational potential energy from the bottom to the top.
M g H = (78 kg) (9.81 m/s²) (300 m) = 229,320 joules .
That's the AMOUNT of work done. Now the question is: Who or what does the work ?
You have to be lifted against the force of gravity, by something ...
some strong men, an army of hamsters, an electrical elevator, etc.
They have to exert an upward force equal to your weight, and
keep it going until you're 300 meters off of the ground.
Whatever it is that lifts you is what does the work.
Gravity certainly exerts a force on you all the way, but the distance
you move is in the direction OPPOSITE to gravity. If you want to
calculate the amount of work done by gravity, using the formula
Work = (force) x (distance) ,
then since the force and the distance are in opposite directions,
their signs must be opposite, and you'd have to write
Work = (m g h) = (78 kg) (9.81 m/s²) (-300m) = -229,320 joules.
The elevator has taken 229,320 joules of energy AWAY from gravity,
and stored it in you. That's now YOUR gravitational potential energy,
as long as you stay at the top. If you let go, or jump, or sail away in
a parachute, or come DOWN the stairs, gravity will DO that much
work on you to bring you down to Earth again.
