The gravitational potential energy is the energy possessed by a mass because of its position in a gravitational field. 

The expression for gravitational potential energy for bringing a mass $m$near the mass $M$is $U=-\frac{GMm}{r}$here $r$is the distance between the mass $M \text{and} m$

Since, $U\propto -\frac{1}{r}$therefore, the gravitational potential energy is maximum at $r=\infty$