Hey,
With the GrovePi, the Raspberry Pi GPIO’s are not connected directly to the GrovePi ports. The GrovePi ports are connected to an atmega 328 and the atmega328 acts as an interpreter. So Python or some other program sends a command to the GrovePi to read data from a sensor, the Grovepi reads it and sends it back to the Pi. All the communications happen over I2C. You can find more about how GrovePI works here: http://www.dexterindustries.com/GrovePi/engineering/software-architecture/ and http://www.dexterindustries.com/GrovePi/engineering/port-description/ and you can find all the sensor examples here: https://github.com/DexterInd/GrovePi/tree/master/Software/Python.
-Karan