How do I calculate the gain of opamps in this circuit?
The DC gain of the instrumentation amplifiers for differential inputs is determined by internal resistors plus Rg. It is a classic 3-op-amp instrumentation amplifier topology. The nominal gain equation is provided in the datasheet. DC gain of the AD8656 stages is +1.
The AD8656 stages form 2nd order low pass filters, and there is single pole low pass filtering on the inputs of the instrumentation amplifier.
Ideally the common mode gain is zero, but it will be affected by the  instrumentation amplifier CMRR at DC and by the matching of the RC networks at the inputs for AC signals.