How to add a Gaussian Noise to a signal so that the noise level on the signal equal a 20%

My signal is a train of rectangular pulses with a temporal resolution of 0.1 s. I want to add a zero mean Gaussian noise so that the noise level on the signal equal 20%. How can I choose the variance of the Gaussian noise so that I achieve this percent? I used the following MATLAB code to generate the signal:

 Signal = gensig('square',20,100,0.1);