As I understand it, it's not a copyright issue, it's a terms-of-service issue. You're only authorized to use GPT in accordance with their terms, which prohibit using the service to develop a competitor. Doing so anyway is unauthorized use of the service, which is a Computer Fraud and Abuse Act violation, independent of what is and isn't copyrightable.
This is an Americanism. In many parts is Europe copyright claims over copyright from the US are honoured, but yet, violation of TOS by itself does not put you in risk of criminal lawsuit.
There are also plenty of places around the world where US copyright is not honored.
This includes, arguably, the US itself, though copyright violation is a lot more blatant in some parts of the world, which don't even offer the fig leaf of trying to enforce US copyright in their legal systems.
