I apologize if folks are tired of hearing me say this, but the 'code' is neither an instruction nor a design manual. Designing 'to code' is a mythical creature, right up there with dragons and unicorns.
While that third wire was added to help ensure that the fuse blew as quickly as possible if something went wrong, it has since been used for a number of 'secret' other uses as well. That's why the code bans 'objectionable' current over the ground wire (EGC); the implication is that these other uses are allowed.
Having a GFCI will address the safety concerns of a circuit without a ground wire - but the GFCI will do nothing else. It will NOT address the other uses that the ground wire is used for.
Having said that, electronics design (the main 'secondary' users of the EGC) has progressed. The use of the EGC for surge suppression, 'phantom' loads, etc., is nowhere as common as it once was.
Plus, our understanding of grounding has improved ... to the point that a lot of the old advice regarding isolated grounds, etc., is now discredited.
My advice? Have a third wire if you can. Do not rely on the GFCI for anything except safety concerns.
