What is earths gravity in gravitational potential energy?

The Earth's gravitational field and gravitational potential energy are really two quite different things. The relationalship is the following:

Gravitational potential energy = mass x gravity x height

Where gravity is the acceleration due to gravity - near Earth's surface, that's 9.8 meters/second2 - or the equivalent, weight per unit mass (which near Earth's surface is 9.8 newton/kilogram).