Disconnect the neutral wire from the load side of the GFCI and check for no continuity between it and the ground wire.
Reconnect the neutral.
Measure the cold resistance of a 7-1/2w incand. lamp. It should be 100 Ω to 400 Ω. Call the value R.
Put the lamp in series with the ground wire that serves the cable downstream of your GFCI.
If the voltage across the bulb reads from 0.004R [0.4v to 1.6v] up to 120vac you've found your leakage path to ground. A normal reading would be 0.0005R for each 100' of Romex downstream of the GFCI.