In order to tell OpenAI to stop crawling a website and thus not use the content for training GPT, OpenAI now supports to disallow GPTBot from accessing your site.
The Information can be found on the GPTBot – OpenAI API website:
Completely disallow GPTBot:
User-agent: GPTBot Disallow: /
Allow/disallow certain parts of a website:
User-agent: GPTBot Allow: /directory-1/ Disallow: /directory-2/
For WordPress sites, I can recommend the Virtual Robots.txt – plugin.