My first CNC machine Arduino Mega controlled!!!!

Does anybody know of an existing Arduino-based G-Code interpreter that is compatible with the Adafruit motor shield? This shield uses it's own library and is not compatible with most G-Code interpreters. including the RepRap one. I tried asking about this in the Adafruit forums, but they seem to be avoiding my question and keep telling me to just use their library, which doesn't include a G-code interpreter.