The gravitational potential due to the earth at infinite distance from it is zero. Let the gravitational potential at a point P be -5J /kg. Suppose, we arbitrarily assume the gravitational potential at infinity to be + 10 J / kg , then the gravitational potential at P will be :

-5 J / kg -15 J / kg +5 J / kg +15 J / kg

+5 J / kg

Potential increases by +10 J / kg every where so it will be +10 - 5 =+5J / kg at P.