i think the gain is most affected by R3. if you would imagine the equivalent circuit of having a pull up resistor (lets say Rp) from the drain of the jfet to +5V.
then Rp and R3 would form a voltage divider therefore, attenuating the output waveform.
again varying the value of the resistor R2 will only change the offset voltage. we would want this to be equal to half the supply voltage (hence 2.5V because the supply voltage is 5V) at when there is no input so the output could swing from 2.5V to 5V for the high side (positive swing) and 0V to 2.5V for the low side (negative swing).
if the value of R2 is improperly selected and lets say that it made the offset voltage be equal to 3.8V, then we would only have a swing of 3.8V to 5V for the positive swing and 0V to 3.8V for the negative swing. because of this, it would cause distortion and more clipping.
i hope i got things correct and clear.