G refers to universal gravitational constant (G) is equal to gravitational force between two objects of unit mass each separated by a unit distance... The value of G is a universal constant ... g-- Acceleration due to gravity (g) is the acceleration produced in an object in free fall due to the gravitational force of the Earth...the value of g is constant at a given place but varies from place to place on the Earth .....The value of (g) is different on the surfaces of different heavenly bodies ,e.g, planets ,moon etc