Hi
If you just want results for some application purpose and this simulation is not specific to just cadence ; then i think you can do this better at MAT-LAB simmulink. Mat lab has built in function for Feedback loop gain.
For 'gain' : It is the measure of ability of a system to increase its output (Power , Amplitude) with respect to Input.
For 'Loop gain' : If this gain is controlled by some feed back then it is called loop gain.
Remember adding Loop to some system decreases its overall gain.
Nyquist Criterion : It tells whether the system under consideration is stable or not and if not stable what changes can be done to make it stable !
This is actually dealing with complex variables in terms of systems ;)
and Plotting results.
thanks