A large ungrounded system can have enough capacitance to ground to provide a fatal shock. A GFCI on such a system should work.
Again, we're talking theoretical, which means generalizing. I did not mean to imply that a hazard can never exist on an ungrounded system. Yes, if a current to earth can be created, a shock can occur.
However, for a GFCI device to trip, there would need to be a sufficient
difference of current between the lines involved and the earth created by the accidental contact, requiring one line to be better coupled to earth than the others.
If all phases have the same coupling to earth, which is necessary to avoid nuisance tripping, no shock current would flow
outside the current sensor. Any current to earth through a person would travel through all lines equally.