The problem is the DC currents cause damage to the track, and it's this that causes the momentary open-circuits when the wiper is moved. Applying switch-cleaner usually improves the situation for a while, but there is no permanent cure. That's why I said above that you will need to fit a new potentiometer along with a DC-blocking capacitor.
Calculating gain is just a case of gm.RL - find gm from the datasheet of the MOS-FET, and when determining RL, remember to account for the load imposed by the following circuitry, which is in parallel with RL. By RL, I mean the 5k1.
This gives you the open-loop gain - that's when the pot is set to minimum resistance. To calculate closed-loop gain, divide RL by the setting of the pot - that's accurate enough for rough purposes.
Interestingly, in this circuit, should the wiper go open, the gain should drop to unity. Did the end of the track get returned to ground, as the schematic shows?
Bear in mind that the DC conditions change when the pot is changed, which would normally be a Bad Thing in an audio circuit, but here, where it's just trying to sound as crap as possible, that's probably an advantage. *shudders*