Calculating Op-Amp Gain on AC voltage
Probably want to tie the 50Kohm resistor (from + input to - input) to ground, thus running from (+) to GND. Otherwise only very high frequencies will be amplified, as the Cin provides a bit of phase shift between Vin- and Vin+ and thus the OpAmp as some input signal to work with.

Now for the gain: if pot's wiper is fully to bottom, the gain is just 24.3Kohm/1Mohm == attenuation by 40:1.

If wiper is 10% from the top (10% away from GND), you still get attenuation, of 4:1.

If wiper is 1% from the top, you get gain of 1/0.4 or Av=2.5x.

How to think about this circuit? {thanks for asking it, because I faced a similar circuit a few years back, but did not need to extract the GAIN MATH).
KEY to understanding: the virtual_ground at (_) input pin. What is the maximum input current, thru the 1MOhm, the OpAmp can control? With output clamped by back-to-back zeners to ~3.4 volts, and with pot wiper fully at bottom, the maximum current availabl from the OpAmp is 3.4v/24Kohm or 140microAmps.

Now assume that current flows past the Virtual_Ground, and into the 1MegOhm.

The I*R value is 140uA * 1Mohm = 140 volts. With only +-3.4 volts on OpAmp Output pin.