the Neuron does not use the GPIOs of RPi for the I/Os. Instead there are 1-3 ARM processors (depending on the S/M/L Neuron version) for each group which communicate with the RPi using SPI, where a kind of Modbus protocol is used.
We are working on the Evok to be finalized and debloated images. Right now there is a pretty simple possibility of using the TCP modbus server (available at localhost:502) which is included in the Mervis img, map of the register is available at http://downloads.unipi.technology
So the easiest way for you might be to get a C++ library for TCP Modbus and get the advantage of the Modbus interface.
Do not forget to stop and disable unneeded seriveces which are included in the Mervis OS:
systemctl stop sharkrt systemctl stop configtool systemctl disable sharkrt systemctl disable configtool