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.
This is my personal opinion and not legal advice. IANA trademark practitioner.
Before the EUTM [0], the EU trade mark office, they're currently in opposition proceedings with no oppositions registered.
AFAIK the term GPT is entirely descriptive (so not distinctive) and so is unsuitable for use as a trademark. It should be refused, but I wouldn't expect trademark examiners to have heard of generative-pretraining, nor that this can be applied to transformers to make a gpt. Unless someone tells them then I expect the application by OpenAI would be allowed.
That said, OpenAI can use the term "trademark" if they're using that term as a mark of origin for goods or services, it doesn't need to be registered first (at which point it becomes an RTM and can use the (R) symbol). Indeed you can register a mark as having acquired distinctiveness through your prior use as a trademark ("acquired distinctiveness").
How do I file an opposition? Oppositions against EU trade mark applications have to be filed in writing within a period of 3 months following the publication of the EU trade mark application in the EU Trade Marks Bulletin. For your convenience, oppositions can also be filed using the online form.
Actually, what I think I was looking for was "observations", oppositions is for other parties with standing (eg other traders, other trade mark owners) I think. Whilst observations, also part of the adversarial processing, https://guidelines.euipo.europa.eu/1803468/1786255/trade-mar..., are for third parties.
Opposition has a fee, and it's probably not appropriate for me personally to be involved (I work in IP, not in TMs). Maybe I'll look at anonymous submissions under Art.5 EUTMR.
(That link says it is obsolete, I'm assuming that the procedure has just been updated.)
IANAL but also there is a specific list of classes (categories) that have to be selected from when seeking to register a trademark, design mark etc and afaik protection then applies only within those categories.
In this case they have applied for Nice classes 9 and 42.
I'm not sure, but also seem to recall that protection within those classes would be further limited to the scope of what they covered in the description they provided for each class during application.
Anyways, point it is that it's in any event really hard to see how having a fairly nondescript domain name with a blank web page could ever be construed as purveying anything in either Nice class 9 or 42.
(Eg, for all we know, such domain name might be referring to some new kind of industrial oil or lubricant, which would be Nice class 4...)
It's also useful to consider that there are contexts where the law can negate the validity of a trademark based on the registrant's lack of actively "practicing" it (including during the period between application and it being granted), or protecting it. So there's an inherent incentive for companies to be a bit "trigger happy" in staking their claim to "practicing" the band as well as acting to protect it. Although it does seem very preemptive in this case.
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...