Adding the resistor and capacitor is a good idea because the capacitor will block any DC voltage from being sent into whatever is plugged into the input or output of the circuit. This protects it from damaging anything if DC voltage was somehow sent to the in/out.
A gain of 2 because (1 + 10k/10k) = 2. (Technically I actually used 2 1k resistors because I had them already out but it works out exactly the same).
Min gain of 1: (1 + 0.01k/10k) = ~1
Max gain of 2: (1 + 10k/10k) = 2
Gain of -2 (-(22k/ 10k) = ~-2) but the wave is inverted.
Min gain of 0: (~0.01k/4.7k) = ~0
Max gain of -2: (10k/4.7k) = ~-2
Min: 8k Max: 1M
With a 4.7k Rin, and the photocell for Rf, the gain will be:
In reality a ton less.