What is the purpose of GFCI?
Circuit breakers trip on overcurrent, including too many devices. A low level fault to ground may not pass enough current to trip a regular circuit breaker.
A GFCI device compares the current on the hot and neutral, if not equal it breaks the circuit. It does this by running both wires through the same current transformer (donut), if the current is equal the magnetic fields cancel each other, and no current is detected by the current transformer. Current flowing a 150% of circuit breaker rating should trip the circuit breaker, but if the outgoing and returning current were the same no magnetic field would be detected by the current transformer and the GFCI would not trip.