In PSPICE how do I get a plot showing the reflection co efficient on the y axis?
You need to find a means to measure input voltage and input current from your circuit.  Then you can convert to complex impedance: Zl = V/I

Then gamma = (Zl - Zo) /(Zl + Zo) where Zo is the transmission line impedance.

You may be able to use an AC current source Into the DUT to find impedance