Apparently OpenAI has been able to register “GPT” as a trademark in EU [0], while their application in US is still under review [1].
[0] https://euipo.europa.eu/eSearch/#details/trademarks/018836652
[1] https://tsdr.uspto.gov/#caseNumber=97733259&caseSearchType=US_APPLICATION&caseType=DEFAULT&searchType=statusSearch