Does it mean I can't form a company and call it let's say "Enterprise GPT"? Or create a product that'd be called "Copilot GPT". Please advise.
I doubt that their application seeking to trademark "GPT" will be approved. Maybe they'll get it for "ChatGPT" or other more specific products, but probably not just "GPT" by itself...
To be able to register a trademark in the U.S., the applicant has to show that the proposed trademark is in fact "distinctive" of their company. The more generic a term is in its field, whether to begin with (i.e., by not becoming distinctive in the first place...), or over time (i.e., by failing to maintain its distinctiveness), the less likely it is to be registerable. And, such "distinctiveness" is notably harder to achieve and/or maintain for terms that are more generic/descriptive rather than truly unique…
In the case of "GPT," in the context of software/AI, those letters -- particularly in that combination -- are understood to stand for things that refer to a kind language model having certain characteristics. OpenAI pioneered the technology, but they don't have a monopoly on it...
Unless you are very wealthy and would like to find the limits of trademark law I would suggest you find another name.