Forget all the stuff out there that says the GDPR protects EU citizens. This is a question of jurisdiction and enforcement. Say I run a blog under a business registered in the US funded by advertisers in the US. A EU citizen that comments on posts issues a GDPR request that I ignore. Their government fines me. I tell them to get bent, I am out of their jurisdiction. What can they do at that point?
As @hamid said: An agreement exists between the US and the EU granting any citizen the right to go to court to defend their rights to their data
There‘s a summary as well as links to the actual agreements here
That agreement concerns use of EU data by the US government itself (ie the intelligence community and law enforcement).
It does not give EU citizens any opportunity to enforce claims against US companies in US courts.