GPIO stations are not defined for OpenSprinkler 3.0 (including 3.1 and 3.2), because ESP8266 doesn’t have enough GPIO pins. In the firmware code, the PIN_FREE_LIST is empty for ESP8266:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/defines.h#L304
The GPIO pins referred to by the GPIO stations are assumed to be pins on the microcontroller, not through IO expander. This can certainly be modified in the firmware to provide support for expanded GPIO pins.