To what extent was Nick truly open-minded?
There is an argument to be made for Nick being, ultimately, open-minded, but this is despite his continued judgments of most of the characters in the story.
Nick is not “objective” as a narrator. For evidence of his subjectivity, the party scenes offer plenty of evidence as Nick describes the events with quite a bit of judgment and value-laden observations.
Though he is not objective, Nick can be said to be open-minded. He attends Gatsby’s funeral after all, claiming this man, Gatsby, as a friend even though at this point he has been thoroughly disgraced, exposed, and to associate with him is no longer glamorous but quite the opposite.
Nick’s tendency to judge can be seen as a narrative measuring device used to weigh and explore the world he inhabits in the novel. The Great Gatsby, in the end, is a novel about identity and the pitfalls of aspiration (both false and genuine aspiration). Only moral judgment can serve to provide the proper color to a narrative focused on these themes.