As wan23 already noted, the purpose of the geoip block is to help establish that you are not offering goods or services to data subjects in the Union, in order to not fall under GDPR territorial jurisdiction (see Article 3).
Yes, that will also catch some of your existing US customers when they travel to Europe. However, you may actually want that, because the GDPR does not talk about EU citizens. It talks about data subjects "in the Union".
I haven't seen anything about what makes someone who is physically present in the EU count as being "in the Union" for GDPR purposes. One who wants to be cautious might want to count anyone physically in the EU as being "in the Union" for GDPR purposes until there is definitive guidance otherwise.
It's an efficient way to protect yourself from the regulation. You don't need to prevent Europeans from using your site, you just need to be able to say reasonably that you don't intend to operate in the EU.
If you can live with infuriating a small portion of your customers while have a protection that can be circumvented in a matter of seconds then geoip block is what you want, otherwise ...