Flag it as "[this answer should be] looked at by a moderator". That's all you need to ever do.
Provide as much evidence as you possibly can to a moderator to explain why you think this is a ChatGPT response.
Don't respond/interact with the person directly if you can help it since we don't want to make this a witch hunt. People are going to react aggressively if you accuse them of being a witch using ChatGPT, so flag and let the mods sort that out.
Don't flag it "for not answering the question" or "as very low quality", because 1) that is not the problem with the answers, 2) these need to be reviewed by moderators, not the Low Quality Answers queue, and 3) if these flags are validated, they will result in extremely confusing audits.
If it's spam (real spam, the unsolicited marketing kind, not just unwanted content), then flag it as spam. Lots of spammers are now using generative AI to produce the contents of their spam posts. But if the post is actual spam, then it should be flagged as spam. The fact that it contains AI-generated content is irrelevant and outweighed by the fact that it is spam.