Neurons aren’t how you measure IQ. The gut system is a primarily a sensory one which simply gathers and relays information, however, there is processing that happens locally so that it doesn’t all have to be relayed to the brain, utilizing similar mechanisms to reflexes (what happens when the doctor hits your knee with that little mallet). At the end of the day your gut is not independent of your brain, thus measuring anything absent the rest of your body isn’t a particularly useful metric.