I highly, highly recommend you read up on electrical theory. When you're done go de-energize a system, disconnect the ground on a GFCI outlet, then connect the live of that GFCI to whatever piece of random material you have. Or lay it on the ground directly. When you re-energize the system, the GFCI will trip or it is bad.
If a GFCI was dependent on a device being grounded it'd be no better than a regular 3 prong outlet. We'd also have to ban 2-prong devices, which are probably most devices in a US home at this point.
If a GFCI was dependent on a device being grounded it'd be no better than a regular 3 prong outlet. We'd also have to ban 2-prong devices, which are probably most devices in a US home at this point.