Or you could step V in a DC OP simulation, Parametric
Analysis, and just plot op(cgg). If you're not set on directly
emulating the measurement setup.
If you set the measurement signal large then your C will be
changing all the time, and since it's nonlinear you can't just
go and average it. Though in AC analysis it's all bogus, if
you ran a transient you'd see C varying across the cycle at
1V amplitude (especially if you're swinging across inversion
threshold). Setting input amplitude to 1 for AC is just a simple
default and often makes the output amplitude be a useful
datum directly (like, if input=1 then output amplitude also
= gain).
I'd be applying transient stimulus amplitude that is maybe 1%
of full span, if I wanted 1% kind of accuracy where things are
changing a lot.