What is gravitational potential energy? What is zero potential energy?

(N/A) Gravitational potential energy is defined as the work done by an external agent in bringing a body of mass $m$ from an infinite distance to a specific point within the gravitational field of another body.
Mathematically,the gravitational potential energy $U$ at a distance $r$ from a mass $M$ is given by $U = -\frac{GMm}{r}$.
Zero potential energy is defined as the state where the gravitational potential energy of a system is considered to be zero. By convention,the gravitational potential energy of a system of two bodies is taken to be zero when the distance between them is infinite $(r = \infty)$,because at an infinite distance,the gravitational force exerted on the body is zero.