I have no idea why you put this under "Biology" but anyways I will try my best to answer this question.
Gravitational energy or gravitational potential energy is always measured from some reference point, where *gravitational energy is assumed to be zero*
Suppose I place a ball having mass m on the floor. Assuming floor to be the reference level(zero potential level), GE associated with the ball will be zero. But as I lift ball to some height suppose h, GE will be *mgh* were g is acceleration due to gravity.
GE varies linearly with height but only to some extent.
