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