What is the formula for gravitational potential energy

gravitational potential energy = mgh

m=mass
g=gravitaional field strength (9.81)
h=height

mass has to be in kg
g is in Nkg^-1
h= has to be in metres

all has to be standard units

mgh if the heights are very small compared to the radius of the planet.
if the heights are comparable to the radius then it is -GMm/r

The force on the object is F = mg and energy = force x distance so

PE = mgh

U = -GmM/R

F = GmM/R^2

F=-dU/dR