Can a ground fault be caused by something outside the home?
A GCFI (or RTD) works along these principles: -



It monitors hot (live) and neutral wires and if it sees a current flowing down one wire that isn't flowing in the other wire (called a differential current) then the current transformer produces a voltage that is used to activate a trip relay (sensor relay). Notice the test button is used to create such an effect by forcing a small current through only the neutral wire.

If water (for instance) is dripping onto an AC outlet (imagine that is the yellow hand in the picture) then current flows through live (hot) directly to ground and does not flow back through neutral hence the current transformer is imbalanced and produces a signal that trips the relay and power is disconnected.

If the fault occurs inside an appliance then try removing the appliances and restarting the GCFI but only remove the appliances with power off else you run the risk of electrocution.