This is from FLUKE corporation, Electronics tester maker. Doug
A GFCI uses a current transformer to detect the difference between the line current supplied to the load and the neutral current returning from the load. Ideally, this difference should be zero because both currents cancel. If there is a current differential, it becomes the input to a comparator within the GFCI, which changes states when the differential is around 6 mA. When the comparator changes states, it triggers a silicon-controlled rectifier, which disconnects the power from the output of the GFCI.