The expression of gravitational constant from Newton's law of gravitation is,

$F=\frac{GMm}{R^2}\Rightarrow G=\frac{F{R}^2}{Mm}$

Value of the universal gas constant is,

$G=6.67\times {10}^{-11} \mathrm{N} {\mathrm{m}}^2 {\mathrm{kg}}^{-2}$ or

$G=6.67\times {10}^{-8} \mathrm{dyne} {\mathrm{cm}}^2 {\mathrm{g}}^{-2}$.

That is, the value of $G$ is not same for all system of units.

$G$ is named as an Universal Gravitational Constant as it is constant at any location of the universe and as well as at any medium in the universe.

 Hence, it does not depend on the nature of the medium in which the bodies are kept.