Gene showed the answer to your question Teddy. His diagram is very accurate as to the external to the GFCI wiring,, I suspect it is correct as to the inside as well.
The GFCI is a teeter totter.. So long as the "Weight" (Current) in the neutral and the hot wire are the same, Nothing happens (it passes power) but as soon as you introduce a 2nd return path (A ground fault normally) the thing trips.
As Gene showed. that path is your body in many cases.
If you are hooked to MAINS power (SHore) then the ground and neutral wires are bonded in several places actually.. At the power teransformer, at the main service box and at the park pedestal. (In theory, in practice, Do not count too heavily on that last one) Thus if there is a hot wire to case short and you touch the case YOU become a return (neutral) path via the ground. Not a good thing, Been there, Done that, Tossed the hand grinder across the room. (I tend to THROW when that happens, most Clinch).
The GFCI does not need, or care about, a ground, It only looks to see that the teeter totter is balanced.
NOW: if you are using a power source that has no ground (IE: INverter not bonded) THEN a ground fault will not trip the GFCI, because the current in the hot and neutral legs remains balanced, there being no 2nd return path.
