Magnitude of the Force F of gravitational attraction between two masses M and m, separated by a distance d, is given by:
F = G M m / d²
where G = Universal Gravitational Constant
The force is directional and is a vector. G is a scalar and is a constant. It is a scalar because it does not follow the law of addition of vectors.
Let G = G1 between masses M1, M. Let G = G2 between masses M2, M. Then the G3 between M1 and M2 is still same as G and not the vector addition of G1 and G2.
G is same in all directions and between any type of masses. Hence, it is directionless. So it is only a scalar.