The API is available and documented for everyone to use. If you wish to control the GPIO directly, you are more than welcome to do so. Also, you may use the existing Python or C++ code to see how stations are toggled. For example, the relevant code is contained here in the Unified firmware: https://github.com/OpenSprinkler/OpenSprinklerGen2/blob/master/OpenSprinkler.cpp#L545-604.