The gravitational field is the space that an object's gravitational force extends. All objects that have mass generate a gravitational force. The greater the mass the greater the strength and size of that gravitational field. Since the Earth is so massive, it overshadows all other gravitational forces in the immediate area. But when we move away from the earth it's gravitational influence is weakened. The formula is:
F=G((m1*m2)/r2)) where: F is the force between the masses, G is the gravitational constant, m1 is the first mass, m2 is the second mass, and r is the distance between the centers of the masses.