F_1 = {Gm_1m_2\over r^2}
If separation between two bodies is doubled then,
F2 = {Gm_1m_2\over (2r)^2}
F2 = {Gm_1m_2\over 4r^2}
F2 = {F_1\over 4}
Therefore, when the separation between the bodies is doubled the gravitational force reduces to one-fourth.