how to calculate voltage gain
the NMOS PMOS combination between A and B seems to like a Class AB control circuit. Also, there is an AC short between point A and B. You can convince yourself by doing proper analysis. Therefore, you can calculate the voltage gain of the amp as you would do regularly (i.e. calculate the gm of the input stage and the output impedance of the first stage and gm of the second stage together with the output impedance of the second stage and so on).
However, what you should keep in mind is that the gm of the input stage changes depending on the common mode input voltage. Gm of the input stage when your common mode is such that both the NMOS and PMOS pair are on is higher than when just the NMOS (or just the PMOS) pair are on.
HOpefully this helps.
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