I dont know about gm-c filter, however if you are trying to find the gm of the input transistor then you could do the following thing.
Make a text file and write this line of code
simulator lang=spectre
save M0:gm
where MO is the input transistor. In the Analog Artist window, include this file as the stimulus file, so that while simulating spectre can pick it up. Now if you sweep some parameters (dc or trans) and look in "Result Browser" then you can easily plot "gm". This process is true for all transistor parameters inside.
I hope it will help you. If so, then please click the "helped" button. Thanks