Yeah, the absence of a terminating resistor on the output, and the lack of any gain feeding the side chain suggested to me it was intended to be an add-on to an existing circuit.
But simple, though.
I've been waving the flag for dual-gate approaches to noise control for a number of years: one at the head of the signal path to clean up any hum coming from the guitar, and a second one at the tail, to clean up any accumulated hiss. High-gain stages/devices generate hiss, but can also amplify hum coming into them. Killing the hum at the outset gives you less to clean up at the end, such that thresholds for both noise-control devices can be set sensibly low and not interfere with hearing the instrument.