F is the force
G is the gravitatational constant. In CGS (centimeter-gram-second units),
G=6.673 x 10⁻⁸ cm³ gm⁻¹ s⁻²
In MKS (meter-kilogram-second) units,
G=6.673 × 10⁻¹¹ m³ kg⁻¹ s⁻².
m1 is nass of the first body and m2 is that of the second.
d is the distance between the two bodies.