Text generated by ChatGPT that you have reproduced should be cited, if for no other reason than the fact that you are not the author. Taking credit for work that is not your own is unethical.
Almost hilarious that ChatGPT doesn't give citations or give credit to anything it writes, despite its entire knowledge being derived from us.
(I am aware there are academic books)
In academic writing, there's a standard that the authors are the sole authors of the text that is upheld by academic institutions, academic communities, and the academic publishing industry (but not the legal system itself). This same standard doesn't exist in the same way in non-academic publishing (or a lot of other media) so not crediting ghost writers is considered acceptable.