After studying the Gravitational force, famous Scientist and Mathematician Sir Isaac Newton formulated a law in 1687 AD, which we know as Newton's Universal Law of Gravitation.
Newton's Universal Law of Gravitation
It states, “Two or more than two bodies, in the universe, attract each other with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.”
\begin{equation} F \propto m_1.m_2 - (i) \end{equation}
\begin{equation} F \propto \dfrac{1}{r^2} - (ii) \end{equation}
From equation (i) and (ii), we get,
$$F \propto \dfrac{m_1.m_2}{r^2}$$
$$F = G\dfrac{m_1.m_2}{r^2}$$
where 'G' is the proportionality constant, called the Universal Gravitational Constant. Its value is constant everywhere for everybody.
The SI Unit of G is Nm2/kg2. Its value is 6.67*10-11 Nm2/kg2.
It is called a universal law because it applies to everybody in the universe regardless of the place, and their physical characteristics.
By Newton's Universal Law of Gravitation, we can obtain the expression for the force of gravitation or gravitational force.