(Sorry for my bad English.) Suppose we choose the surface of the Earth at a point as having a zero potential energy. Now if there're two particles of equal mass at rest, one of them just an heigh above the another, the one which is at the ground will not have potential energy. Since the particles are at rest, their energy would be equal to mc². Since the particle above the ground will have a greater energy, it would have a greater mass. Then is it correct to say that the gravitational field gives mass to the particle in that situation?