How gain is changed in this Op-AMP integrator?
When the input is negative, the op amp output is positive. D3 is ON.  The circuit is an inverting amp with a gain of 1.4. When the input is positive, the output is negative. D2 is ON. The circuit has no gain. The op amp output is then a half wave rectified signal. R5 and C3 are a low pass filter. C2 also acts as a low pass, when the op amp has positive outputs.