$F=G\frac{{{m}_{1}}{{m}_{2}}}{{{r}^{2}}}\Rightarrow unit\{G\}=\frac{N{{m}^{2}}}{k{{g}^{2}}}\xrightarrow{N=kg\frac{m}{{{s}^{2}}}}unit\{G\}=\frac{{{m}^{3}}}{kg.{{s}^{2}}}$