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Question
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Gauss's Law is valid only for symmetrical charge distributions.
Gauss's Law is valid only for charges placed in vacuum.
The electric field calculated by Gauss's Law is the field due to the charge inside the Gaussian surface.
The flux of the electric field through a closed surface due to all the charges is equal to the flux due to the charges enclosed by the surface.
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Solution
The flux of the electric field through a closed surface due to all the charges is equal to the flux due to the charges enclosed by the surface.
The contribution of flux on the closed surface due to the charges lying outside the surface is zero because number of field line entering the closed surface is equal to the number of field lines coming out of the surface so the net contribution of the charge lying outside the closed surface to the flux is zero. Therefore, the net flux through the surface due to the charge lying outside the the closed surface is zero. The contribution that counts is only due to the charges lying within the closed surface.
Thus, the flux of the electric field through a closed surface due to all the charges (inside and outside the surface) is equal to the flux due to the charges enclosed by the surface
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