It applies to EU citizens globally. If you store data on EU citizens -- you're expected to comply with GDPR. This will be ineffective for small companies in the US or China but nearly all large companies will have a presence in the EU.
It applies when you process data about people physically in the EU (whether they're citizens, residents, on holiday or just transiting through) or if the person/company doing the processing is based in the EU.
"The protection afforded by this Regulation should apply to natural persons, whatever their nationality or place of residence, in relation to the processing of their personal data."