Why is Principle of superposition not oftenly used for gravitational potential? We talk a lot about this principle for electric potential at point by many charges but not for gravitational potential by other masses around it, why?
 A: 
We talk a lot about this principle for electric potential at point by many charges but not for gravitational potential by other masses around it, why?

Here is a statement of the principle:

According to the principle of superposition, if each of these interactions acts independently and uninfluenced by the other bodies, the results can be expressed as the vector summation of these interactions;

$F = F_12 + F_13 + F_14 . . . . . . + F_1n$ .
The force on particle 1 of all the other forces.

It states that:
“The resultant gravitational force F acting on a particle due to the number of point masses is equal to the vector sum of forces exerted by the individual masses on the given particle”.

On earth, the collective gravitational field, which is a superposition of the individual masses of zillions of particles, is treated as one gravitational potential, measured at the point of earth as we need it. The superposition principle is implicit . In planetary models, where the planetary bodies can be used as particles with a given gravitational field, the superposition will be explicit in the equations.
