This is hypothetical and it's only meant to ask about the extent of the law, not ask for a particular legal advice.
Let's say a US merchant's website uses exclusively regional US banks. Because they are regional, the banks can provide all electronic services of any national bank, but do not have any international presence.
If any of the visitors to the site happen to be from outside of the US, a disclosure appears stating that the website elects to NOT COMPLY with GDPR. This may get the operators of the site in legal trouble in the EU, but they are confident that they remain on solid legal ground within the framework of the US law. And they are also confident that, as a result, GDPR is unenforceable against them.
Can the site's payment processors (or any other vendors) be liable if any legal authority in the EU decides to pursue this case in order to establish a legal principle or even to simply make an example of the site's non-compliance?