Hi all, In xschem, How I can calculate or get Gain of circuit. @Stefan Schippers do you know?

Do you need small signal gain? first set the correct operating point of the circuit, then apply an AC voltage source to the input with unit amplitude and zero phase (Vin In_node 0 AC 1 0), run the .ac analysis and plot the dB and Phase of the output. ngspice user manual:

15.3.1  .AC: Small-Signal AC Analysis

it is also possible to know the gain of a circuit in doing a DC voltage variation in the input with different several polarisations voltage. It is difficult to be more precise without more details.