I think these domains are a reasonable investment. I really doubt that OpenAI's application to trademark "GPT" would be approved...
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. 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 (specifically A.I.), those letters -- particularly in that combination -- are understood to stand for things that refer to a kind of A.I. language model having certain characteristics, even though OpenAI was first to produce a (g)enerative (p)retrained (t)ransformer and they're still the most notable provider of such technologies.