the current on the ground wire would not in it self be dangerous, the danger occurs when the current leaves the wire or device and travels through you to ground.
the GFCI absolutely without doubt measures and compares the current in the hot/neutral (if you want to call it that) and respondes to an imbalance between the 2 wires of 6 mAmps.
here is a link:
http://www.google.ca/imgres?imgurl=h...ed=0CCYQ9QEwCA