We've posted two conflicting answers to this question and I think that's indicative of how little the law is understood, only a few weeks from going into force.
Just because people on messageboards are posting different things, doesn't mean the law is unclear. GDPR Art 3 is relatively clear that it wouldn't apply just because an EU citizen is in the USA. Once that person is in the EU, then it applies.
Well I didn't necessarily say the law itself is unclear, just that the understanding of it as a whole seems to be unclear. Anecdotally, almost everyone I speak to at events seems to have different views on what they need to do. I'm in the EU.