The gravitational potential energy is the work done or energy transfered per unit mass which is required to move an object to that location from a fixed reference point.
Schematic diagram:
The gravitational potential energy of the pendulum bob having mass m relative to its center position is given as,
Here, m is the mass of the bob pendulum, g is the acceleration due to gravity, and h is the height from surface.
Therefore, the gravitational potential energy of the pendulum bob relative to its normal center position when it is first released is mgh.