Dear student,
Gravitational potential energy: the Gravitational potential energy of the body at a given point is defined as the energy stored in the body at that point. If the position of the body changes on account of the force acting on it, then change in its potential energy is equal to the amount of work done on the body by the forces acting on it.
It's S.I. unit is JOULE (J).
Gravitational potential energy is negative. So its maximum value would be zero and that too at infinity.
Regards