The electric flux Φ = ∫ E.dS

and according to the gauss theorem Φ = q / ε_o

Here, q is charge enclosed by gaussian surface, dS is area of gaussian surface, E is electric field.

So, for every derivation based on gauss theorem, you have to just equate the ∫ E.dS and q / ε_o

So, calculate the ∫ E.dS and net charge in surface and then put ∫ E.dS = q / ε_o