Why is capital G a constant?

Dear Student ,
G is constant because it is known as the universal gravitational constant and also its value does not change with change in medium and on other planets the value of G remains same. This is the main reason behind we say G is a constant .
G = Gravitational Constant is constant physical quantity with a constant value .$G=6·67\times {10}^{-8} dyn·cm/g{m}^2=6·67\times {10}^{-11} N·m/K{g}^2$
Regards