First of all, I notice in your frequency response - a peaking response. It would lead to instability. Forget about correct gain value. Try to eliminate that peaking.
Second: Check whether all the transistors are in saturation. How to identify it? Well, apply only the DC voltage(s) and check the voltages at all the nodes and you know the condition for checking whether the transistors are in saturation. Note that for short-channel, it is slightly difficult, or rather little time-consuming. Then check whether you have appropriate current in the bias transistor, and both the input transistor. Once you know those details, check the same for 0.5v above and below that DC value. then set the AC signal for 1/100000v and do the AC analysis.
the foregoing points may help.
Post the W/L for the circuit, anyways. It may help.
- srivats