How to calculate the voltage gain in this picture?
The voltage gain is 0, because \$V_{out}\$ is connected directly to a voltage source, so its voltage will never vary, no matter what signal is present at the input.

Furthermore, M2 will probably catch fire, because its gate and drain are connected directly to the Vcc supply, which will cause a large current to flow through the FET, while it also must drop the full Vcc voltage.