It uses its magic crystal ball, while simultaneously consulting a legion of captive demons to determine this and other similarly unknowable information.

I suppose you have to use your TOS for that. (In fact, a banner that tells European users that they aren't allowed to use your site is probably the easiest way to insulate yourself -- if you collect their data because they used your service illegally, I'm not sure you can be blamed.)

You misunderstand the GDPR if you think you carry its protection around when you visit non-European locales.

It doesn't. It just checks that somebody is connecting from an IP address geolocated in Europe.

I tried to be sarcastic, but I think my English is not good enough :-)