The gravitational force between two bodies forms an action-reaction pair between them.                          

The gravitational force is a force of attraction between any two bodies that have some mass given as:

${\text{F}}_{ }=\text{G}.\frac{{\text{m}}_{\text{1}}{\text{m}}_{\text{2}}}{{\text{r}}_{ }^2}$
Where m₁ and m₂ are the masses of the two bodies, r is the distance between them and G is the universal gravitation constant.

Both bodies exert an equal force of attraction on each other in the opposite direction. 
For example, the earth exerts a gravitational force on the moon that is directed towards the center of the moon, and similarly, the moon exerts equal and opposite gravitational force on the earth.

Hence we can say that the gravitational force between two bodies from the action-reaction pair that acts along line join the two masses.