Gravity-gradient stabilization uses the Earth's gravitational field to keep the spacecraft aligned in the desired orientation. The spacecraft is designed with a mass distribution (e.g., two long booms) that keeps one end closer to the Earth. Gravity-gradient techniques take advantage of the decreasing strength of the gravity field (the gradient) as you move away from the Earth. The part of the spacecraft nearest the Earth (on one end of the boom) is in a slightly stronger portion of the gravity field. This technique has been used in low Earth orbit. The question is whether or not the gradient is too weak to be useful at higher altitudes.
Electrodynamic tethers utilises this force to maintain its position.