If you have the S-parameters as complex numbers s, then the impedance Z depends on the impedance Zo of the line used for the measurement, probably 50 ohms.
but i use the equation for microstripe line 50 ohm. in my work, the Zin is producted from parasitic parameter into transistor. do you are sure that can use the equation in gate and drain of the transistors?
It's not my area, but I think each port should have its own s-parameters, that correspond to the impedance in that port. There should be S11 (SGG?) and S22 (SDD?). S21 (SDG?) will be the gain, I think. I may not have understood your question.