Is the narrator reliable in The Picture of Dorian Gray?
I think you might be getting confused between different points of view here. Normally we only associate questions concerning the reliability of a narrator with tales that are told using the first person point of view, when one of the characters in the text tells his or her tale. Clearly, such a point of view is partial and not complete, and depending on the character, such accounts will be, to differing extents, unreliable.
However, this excellent novel has an omniscient narrator, which means they "know everything" and have access to the thoughts and desires of every character. We can associate the omniscient narrator with having a kind of God-like view. They are exerior to the action and do not appear as characters in the text, and have unfettered access to all characters. Thus, questions of reliability do not enter into the discussion of the narrator of this story. Omniscient narrators are on the whole considered reliable. We trust them because of the God-like vantage point they have.