everything close enough to Earth to be considered in a uniform gravitational field accelerates at 9.81 m/s^2 toward the Earth. Crudely, Force=mass times acceleration so the force exerted by an object as it’s weight is it’s mass times the acceleration due to gravity.
GPE is a special case of the work done by a force, so the work done against/by gravity is the force (weight) times the distance moved by the force (the height). in this case the weight of the box is 0.8*9.81 and the distance over which gravity does work on it is 1.8m.