G is gravity whose value is 9.8 m/sec2 whereas G means gravitaional force which given by G=(g*M1*M2)/(R*R) : where g is gravity;M1 is the mass of body : M2 is the mass of Earth : R is radius of earth.
G is acceleration due to gravity at a place. it can change according to place.G is the universal gravitational constant which remains constant everywhere.
Sorry there was some mistake in the answer that i posted .g is acceleration due to gravity whose value is 9.8 m/sec2 whereas G means universal gravitational constant which which is constant everywhere