PIR Op amp circuit voltage gain question
Don't be scared about this rather simple circuit. C9+R3+R12 and C5+R13+C8 is just a high pass filters. R1+R12 and R7+R8 form voltage dividers to offset X3 (non-inverting amplifier) and X1 (inverting amplifier) respectively. C3 and C7 give a little bit of integrating abilities to them.  R3+R12 also forms voltage divider that will affect voltage gain. Q1 almost do not make any voltage gain because it just an source follower. So, overall gain will include voltage divider (R3+R12), gain of non-inverting amplifier (X3) and gain of inverting amplifier (X1) if signal lays in pass zone of all filters:

G = [R12/(R3+R12)] x [1+R5/R4] x [-R9/R6] ≈ -4604