Mechanics

The work done by an object by a force is

W = F x d

but

F = m x a

so

W = m x a x d

If the object is in free fall, then a = g and the distance is its height, d = h.

Thus the work done by gravity on the object (GPE) is

W = = GPE =  m g h

When a skydiver jumps out of a plane we consider the forces in 3 stages:

Immediately after jumping: There is zero drag, the only force acting is the weight and the acceleration is a = g = 9.81m/s^2

Before Terminal velocity is reached: The drag force increases and so the net force decreases  this means that a < g

At Terminal velocity: drag = weight and the acceleration is zero. Falling at constant speed.