A GFCI doesn't filter anything. It measures the current leaving the device on the phase wire (black) and the current returning to the device on the neutral wire (white) and then compares the two. In normal operation they should always be the same since electricity flows in a closed circuit.
If the two current measurements differ by more than a prescribed maximum amount (4-6 milliamperes), a contact inside the GFCI opens and shuts off the current flow.
So if 9 amperes is traveling through the heating elements in the toaster and 1 ampere is flowing up your arm, through your heart, down your other arm and into the faucet on the kitchen sink, that 1 ampere (1,000 milliampere) difference sensed by the GFCI will open the circuit at the device rather than stand there cooking yourself to death.
