Hi,
The definition of zero as i understand is that frequency where the forward gain due to both the forward paths (as in our case) add upto zero.
You have asked how does gm come into picture, as I understand.
So if we add up the gains through both the paths equate it to zero, we would get the frequecy of zero. Which turns out to be gm/Cc.
As this frequency is very high, the cap Cc more or lessappears to be short and the gate appears to be connected to drain. More or less a MOS in diode connected configuration.
Why do you say that zero frequency is not attainable, it is a function of capacitance between gate and drain.?
This in pratical circuits adds a RHZ that spoils the phase margin, if the frequency as you suggest if unattinable, it should not effect the phase margin at all?
The load shorting at zero frequency is a function of the capacitance at that node, In a thought experiment that could be far smaller than Cc?