If you don't have a precision VNA to measure it directly, then create a condition where the desired parameters would be emphasized or exaggerated.
Example: gate impedance is high, so match it to a low impedance where you can use, say, 50 ohms to deal with it normally.
This can be done at a single frequency with an L match network. 50R < gate Z, so this will look like a series resonant circuit, with the gate on top. That is: 50R source - series L - parallel C || gate.
The resonant impedance, sqrt(L/C), shall be placed halfway (geometrically speaking) between 50R and gate R.
You don't know gate R to start, but you can reasonably assume it's high, like 100s of k, or megs. So sqrt(L/C) should be in the >1kohm range.
1/(2*pi*sqrt(LC)) of course is the resonant frequency. You can only do this measurement at a given frequency, so if you want a range of values, you'll need new component values for each one.
First, quantify this network without the gate. Use a high Q inductor. Measure the impedance dip at resonance. The impedance gives R, and the resonant frequency gives L and C. Note that you can't probe the voltage at the capacitor, because your probe will load it -- though you might find it illuminating to repeat the measurement with the probe connected, to see what effect it has.
Whatever load you attach, repeat the RLC calculation, then separate the R into series (known inductor loss) and parallel (unknown load) components, and the parallel (un/known) components of C.
Note that this must be done at very small signal levels: 10mV at 50 ohms becomes 1V at 500kohms.
It is very reasonable to calculate or simulate the circuit properties, based on datasheet values. You'll at least get within the tolerance of the part itself, which is pretty awful, so you'll need to adjust things later anyway.
Tim