Plotting G(t) of an oscillator in Cadence
The gm is a small signal parameter which is only calculated and available after a DC operating point simulation. Note that an AC analysis first does this DC operating point calculation as it needs that gm because an AC analysis uses the small signal model for the transistors.

Your transient simulation uses the large signal behavior of the NMOS so gm isn't even calculated as it is not needed.

As gm = dId/dVgs you could pick a point on the transient curves of Id and Vgs, clip the curves to you only get a small part and then determine the derivative.

However that will include some currents you might not want to see, for example the current through Cgd and Cdg which aren't really part of gm.