Where to connect DB9 Ground Pin on a Serial RS232 connection?
I don't know the STAMP controllers but in this context 0V = GND. All voltages are relative to some reference point. In this case it is 0V on one device and GND on the other device. They just have different names for the same reference point. 

If you start going in to mains electrical then 0V may not be GND as this could be Safety Ground (referred to as GND often )  and 0V could be isolated so be careful.