Lets say your visiting the USA as an EU citizen and you get a pizza delivery from a local small pizza shop. They put your name and delivery address in their computer in an MS Access database that makes stickers, emails the delivery guy's gmail account and a person delivers a pizza to you.
They have no idea your an EU citizen and they just put enough information into their computer that would violate the GDPR. They have no real way to comply unless they retrofit their computer system to some vendor that is GDPR compliant, if it even exists. Just deleting your info from the msaccess db and asking the delivery guy to delete their emails isn't enough for the GDPR. And a computer system retrofit for most business might as well be like asking them to retrofit their building as far as costs go.
The end effect might be just outright banning all EU citizens from doing business with various places, because it's just not worth the hassle.
'Sorry you cant stay at our hotel, we are not GDPR compliant'
'Sorry we won't deliver to you, we are not GDPR compliant'
'Sorry you can't enroll in our classes, we are not GDPR compliant'
'Sorry we won't treat you at this hospital, we are not GDPR compliant'
'Sorry you can't get a bank account with us, we are not GDPR compliant' (Like a lot of EU banks with US citizens with FATCA)