Two things mixed up here:
The main difference between the typical RCD and a GFCI is that the first is typically passive. It doesn't contain much electronic stuff and would work at 20 VAC as well as at 230V as long as more current is passing than allowed and written on it. On the other hand this principle limits trip minimum to 30mA for 3phase and 10 for 1phase devices.
The generator issue is something else. The very small gennys often have isolated conductors. There is no "hot wire" measured against the ground. It can be compared with "separating transformers" which we use here in Germany f.i. in tv repair shops. This issue is limited by a maximum power of some kW and a maximum length of the isolated mini-"grid". Otherwise a capacitive coupling (correct word?) could produce dangerous voltages against ground also with an isolated system. These limits are well defined in the IEC regs as far as I remember.
