The value is derived from observations by people more knowledgeable in
this stuff than you can even approach.

That is not an answer to my questions. I have never seen any
difference is the measurment of the lightspeed due to gravitational
influence by your scientists.

But I have seen what your scientists claim to be empirical proof on
that time flows slower with increased gravitational influence.

If that is true, then the lightspeed must also decrease with
increased gravitational influence.