why does the meduim does not affects gravitational constant "G"
I think you are asking about some sort of attenuation effect on gravity passing through a medium (like speed of light through glass)?
We wouldn't be able to tell if it did. It's incredibly weak so any effects would be virtually undetectable.
If you are comparing it to the dielectric susceptibility, then there is a similar effect of course. Gravitation will move objects so objects even further away will experience a modified attraction due to the rearrangement of objects.
But this is no negative gravitation, so you don't get the equivalent of the polarisation effect for dielectrics.
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