The GDPR's scope is based on geography, not citizenship. If you're an EU citizen currently in the US, you do not necessarily enjoy whatever rights and protections the GDPR might offer you if you were within the EU. If you're a US citizen currently in the EU, you do enjoy those protections.
It's a common misconception, and has been widely reported even in the mainstream media. FYI it's Article 3 of the GDPR that specifies the territorial scope authoritatively.
Fun fact: The word "citizen" doesn't actually appear in the GDPR at all.