Measuring transconductance of any circuit
AC analysis is a small-signal analysis. It computes the small-signal response on the output(s) when applying a small perturbance on the input(s) around the DC operating point. I do not know exactly how the simulator does it. As your tb looks now the AC source will set the DC value of Vin, which is probably not desired(?) when you have the feedback to set an operating point. Rather put the AC source from the large C to the inverter input. Or, without changes, I suppose you could in the AC analysis sweep the DC value of Vin at a fixed frequency. 

Also, I would recommend to replace the RC load with a voltage source. If you always want to simulate around the operating point you can for instance use a dependent V source controlled by a replica circuit to set the output voltage and measure the output current. That method should remove any dependency you might see from the load.

Here's an example of what I meant with the replica circuit setup :







simulate this circuit &ndash; Schematic created using CircuitLab