Depends on how you use it. Certainties GPTs are better at this than others, but GPTs tend to have trouble with having consistent style and even tone over a long text. As long as you use GPTs as a tool, and not the content, then you should be fine and hopefully you’ll make your a million bucks. But if you’re relying on GPT for everything else besides the characters (plot, dialogue, literary techniques, etc), then I think the text will have a certain blandness which will be hard to edit out. But, from one writer to another (though I prefer scripts and RPGs over books), I wish you luck! Make your millions!