Help with (CNLS) Fitting Gauss Newton

My project started off with me taking a simple RC series circuit and RC parallel circuit and finding the admittance and impedance equation. After which I had to derive the equations and separate real and imaginary, then write it as MATLAB code measure the frequency from 1Hz to 1MHz and plot. Now I haft to use Gauss Newton for CNLS fitting and I don't know where to start. This is my first time really using MATLAB, I'm enjoying it but its getting difficult to understand. Any Information would be a great help. Thanks

This is the Impedance parallel code I used: Resister 100k Capacitor 10e-6

>> R=1e3;C=10e-6;

>> Tau=R*C;

>> f=[1:1:1e6];

>> OmegaTau=2*pi*f.*Tau;

>> Re=R./(1+(OmegaTau).^2);

>> Im=R*OmegaTau./(1+(OmegaTau).^2);

>> plot(Re,Im)

>> title('Parallel Circuit Impedance Plot of Real vs Imaginary')

>> xlabel('Real part of circuit impedance')

>> ylabel('Imaginary part of circuit impedance')

Frequency for this plot is stepped from 1 Hz to 1 MHz in steps of 1