If question is in what source is G universal
Can we say that it is universal in law of gravitation

Dear Student
 

The Newton's constant is just a number which is used to remove the proportionality between the force term on one side and masses and distance on the other side in the gravitational equation. So, it has the same value throughout the universe (clasically anyways).

 
The constant of proportionality is G  that is universal gravitation constant.
The value of G was determined by Lord Henry Cavendish using a torsion balance.

Regards