A GFCI does not require a ground to operate but has ground available. The GFCI senses an inbalance in current between the hot and neutral wires of an appliance. It'll disconnect mains power to the appliance with as little as 3mA in less than a cycle -- so quick that you can trip it by touching the hot wire and never know a current was flowing through you -- not to say that I'd recommend that as a test, of course!
In the U.S. for all new construction, 3-wire, ground outlets are mandatory everywhere and in addition, GFCI-protected outlets are mandatory in kitchens, bathrooms, outdoors ... and now I believe they've started including basements and garages. In general, if you can easily touch a grounded item (earth, sink, faucet, etc.) any outlet has to be GFCI-protected. The one spot where it's not required, which really confuses me, is in a utility room where you have all sorts of grounds available with electrical panels, plumbing, clothes washer and dryer, freezer, etc. In fact, in this country, the National Electrical Code demands that in mobile homes (or modular homes as they're wanting to call them now) 240V kitchen ranges and a clothes dryers are required to have 4-wire connectors: hot, hot, neutral, ground; however, in "stick built" (wooden, brick, etc.) homes, they are only required to have 3-wire (no ground) connectors, which I find to be totally unacceptable.
Dean