I think I am misunderstanding something elementary. Supposing a scenario where you have a negative charge enclosed in a Gaussian surface, why would the presence of outside charges not affect the flux?
In scenario 1 there are no outside charges, so there will be field lines coming in from infinity and none going out so total flux is negative, which makes sense. But in scenario 2 where you have multiple positive charges outside the Gaussian, do field lines from these charges not end in the negative charge in the Gaussian? And the more positive charges you have, the more field lines that end in the negative charge? I don't understand why the flux is the same in both scenarios.