If someone publishes GPL licensed code on github, and OpenAI then modifies that code in some answer it provides to someone asking a coding question, then OpenAI is in violation of the GPL license if they don't also license their stuff under GPL.
Just because an entire industry has their entire future riding on courts eventually deciding that "Yeah, but copyright doesn't apply to AI", that doesn't make it so.
I've read a lot of code in my career, much of it GPL. When I now write code, it's based in part on that previous code reading. I think most people agree that doesn't mean that all code that I write must be licensed under GPL. In other words, they agree it's not derivative. Even if I write a C strcpy implementation that's functionally identical to one in glibc, I think most of us agree it's not derivative, even if I've read glibc's implementation before. Or if someone asks me "how can I write strcpy in terms of memcpy and strlen?" and I answer them with code I write that's very close to glibc's implementation.
Is AI-written code fundamentally different from bio-intelligence-written code in that regard? (I think the answer is uncertain in terms of "how should it work?", not that it's clearly one way or the other.)