I'm interested in this too, it would be especially interesting to use GPIO to control an external relay. But no one says anything here...
GPIO pinout
M33 pinout is shown below:
All output pins are open drain.
Digital input voltage is 0 to 3.63 V max.
To receive a logical "0" on the pin, the voltage should be between 0 to 1.4 V.
To receive a logical "1" on the pin, the voltage should be between 1.41 to 3.63 V.
By default pins 12,13,15,16 are configured for the second "serial1" port usage:
/system/routerboard/settings/set gpio-function=serial1
They can be resigned for GPIO usage in /iot/gpio/digital menu with following commands:
/system/routerboard/settings/set gpio-function=""
/system/reboot