I used the small circuit board as found on eBay. A search of "SSM2167 Microphone Preamplifier Board Preamp COMP Compression Module DC 3V-5V" will show you the module I used.
I simply connected the input to the mic, added a 4.7Kohms resistor between the mic input and the 5VDC (taken from the Raduino) for biasing the electret and put a 10Kohms potentiometer in the output to adjust the power level to the mic preamp stage.
So I didn't modify the uBitx board, simply inserted in the circuit. There is most likely too much gain which is reduced back with the output potentiometer , but I haven't received any negative feedback about the compressor except when I pushed the output potentiometer too high.
I have since removed the "R1" resistor and replaced it with a 51K Ohms resistor to get a 4/1 compression factor, up from the 2/1 as delivered. I haven't tested that on air yet.
On your Bitx40 what values resistors do you have for compression and noise gating?
All the best,
73, John (VK2ETA)