There are a couple of elements at play here. I am not sure that the opening pages warn the reader against Nick. They depict a narrator that is human and contains the frailties of a human as he catalogues what it means to be human. Fitzgerald constructs a narrator that is uniquely and distinctively no different than anyone else, making him a real and vibrant individual:
Only Gatsby, the man who gives his name to this book, was exempt from my reaction — Gatsby, who represented everything for which I have an unaffected scorn. If personality is an unbroken series of successful gestures, then there was something gorgeous about him, some heightened sensitivity to the promises of life, as if he were related to one of those intricate machines that register earthquakes ten thousand miles away.
Such a depiction of Gatsby is one in which there is not a warning against the narrator. It becomes clear that concepts such as "unaffected scorn" and the idea of "something gorgeous" can coexist when assessing human beings. Nick becomes a reliable narrator in that he is no different than anyone else. At the same time, it becomes clear that Gatsby- both as a person and image- is unlike anything we have ever seen. Nick becomes our guide towards revealing that which is both distinct and, upon reflection, brutally human. It's a tension filled dynamic and Nick is our guide through it.
The other issue here is that if one can make a case for the narrator being not entirely reliable, it only reflects what Fitzgerald sees as the essence of human beings. Fitzgerald constructs a work in which no one really escapes. Those who preyed on Gatsby and then abandoned him are treated in a harsh manner. Those who did not do anything to help are equally find themselves heaped with scorn. At the same time, the reader recognizes that there is a reflective element about their own lives at play in the text. The reader does not escape for they are left to assess whether they have encountered anyone like Gatsby or those who use and discard him. Fitzgerald does not want to make anything "clear" because it is clear that he wants the reader to make a voyage inward, reflecting upon both what is in the text and what the text triggers within the individual. Such a explorative quest diminishes if he were to construct absolutes in the opening of the text. This ambiguity is where literature's journey within the reader can take place and a domain where Fitzgerald, as the writer, is able to communicate with the reader both within the text and outside of it.