IANAL, and if you want to stand up to them you probably need one.
However, note that they do not use the word trademark anywhere. That's because they do not yet own the trademark on GPT [0], it's still awaiting examination. I'm not an expert here, but this letter feels to me like they're starting to try to intimidate people into following their guidelines as prep work to ensure that it doesn't become generic before they even get the trademark.
Again, IANAL and I'm not giving advice, but this is not a trademark infringement warning because there is (as of yet) no trademark.
However, note that they do not use the word trademark anywhere. That's because they do not yet own the trademark on GPT [0], it's still awaiting examination. I'm not an expert here, but this letter feels to me like they're starting to try to intimidate people into following their guidelines as prep work to ensure that it doesn't become generic before they even get the trademark.
Again, IANAL and I'm not giving advice, but this is not a trademark infringement warning because there is (as of yet) no trademark.
[0] https://tsdr.uspto.gov/#caseNumber=97733259&caseSearchType=U...