I don’t have a GreenIQ so can’t test. But if you want to get the unified firmware to compile and run on it, I think you just need to modify the pin definitions:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/defines.h#L448
to pins that are not used by GreenIQ. I don’t know if there is a circuit schematic of GreenIQ (or at least a document about what pins it uses). If you change the firmware’s pin definitions to dummy/unused pins, the firmware should compile and run just fine, and then you can use the GPIO zone feature to define a zone attached to a GPIO pin.