Gravitational force, $F = \frac{GM_{1}M_{2}}{R^{2}}$ $\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ G = \frac{FR^{2}}{M_{1}M_{2}}$ $\lbrack G\rbrack = \frac{\left\lbrack MLT^{- 2} \right\rbrack\left\lbrack L^{2} \right\rbrack}{\left\lbrack M^{2} \right\rbrack}$ $\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \left\lbrack M^{- 1}L^{3}T^{- 2} \right\rbrack$