I think you can access the transistor parameters such as gm from the calculator.
So you could do a DC sweep of your common mode and plot the gm.
From the Calculator, open the browser. If you did a DC sweep, you will have in the tree:
psf -- dc-op --
Then select the transistor instance and it's gm.
Or you can access more directly by using the "OP" button. Select the transistor and you can choose to plot the gm. It will be displayed as a function of the DC sweep variable.
Hope it answer your question.