MrAl, a GFCI only requires H and N. In fact they are allowed in 2 wire circuits if properly labeled "No Protective Ground"
Here
**broken link removed** on page #6, PDF page 7, is a GFCI circuit. Do you see a ground in there? I don't.
The subtraction is done with two coils wound together so they subtract. Another winding amplifies this.
Another coil senses a grounded neutral and it does this without a ground. In general the GFCI detects the wrong polarity too.
And if you don't believe me:
http://www.nachi.org/forum/f19/gfcis-2-wire-circuits-38554/
There's a two prong one on my hair dryer with a test and reset button.