Does earth fault current return to transformer's neutral via earth?
Power distribution practices may vary between countries.

In my area (and probably most of US and Canada) the power company's Neutral conductor is connected to an earth ground rod at each step-down transformer, and at the service entrance for each customer.  With this system, a leakage from a hot wire in my home to earth (through a water pipe, for example) will get back to Neutral via the Neutral/Earth connection at my service entrance, and also through the Neutral/Earth connections at nearby power poles.

I consider the name "Ground Fault Circuit Interrupter (GFCI)" used in North America to be slightly misleading - the GFCI will trip if the hot wires (or Neutrals) of two circuits are shorted.  Anything that causes the Neutral and Hot currents to be different will trip the GFCI.  Earth leakage need not be involved.