Why does Nick Carraway in Fitzgerald's The Great Gatsby reveal rumors about Gatsby rather than facts?

In the first few chapters, Nick tells us no more than what he knows about Gatsby.  At this point, all he knows are the rumors that he hears from other guests at Gatsby's parties.  Nick is an honest person, and doesn't want to give us the impression that he knew about Gatsby any earlier than anyone else.

In Chapter 6, Nick finally lets the reader in on Gatsby's lower-class background and pretensions to greatness.  Nick tells us that he himself did not find out all this about Gatsby until "very much later ... at a time of confusion, when I had reached the point of believing everything and nothing about him." In fact, Gatsby does not tell Nick the true facts about his own life until just after Myrtle Wilson is killed, which comes near the end of the novel and shortly before Gatsby's own death.  Why, then, does Nick put this information about Gatsby in Chapter 6, at a point in the narrative when he did not yet know it himself?

Nick, who is not very sympathetic to Gatsby, tells us that he shares Gatsby's story now in order to "explode those first wild rumors about his antecedents" ... in other words, in the interest of truth.  Nick is a moralistic person and doesn't like the idea of someone pretending to be something he is not.  He chooses this moment to share the story, because in Chapter 5 Gatsby has just been reunited with Daisy, and then there is a halt in the action for "several weeks," when Nick does not see Gatsby. 

So I take advantage of this short halt, while Gatsby, so to speak, caught his breath, to clear this set of misconceptions away. 

That is the motivation of Nick, the narrator of the story, who is also a character in it.  But the author, F. Scott Fitzgerald, had his own reasons.

This is Fitzgerald's masterful way of maintaining just the right amount of tension in the novel.  For the first part of the book, Gatsby has to be a man of mystery.  The reader has to feel the same fascination and curiosity about him that is felt by Nick and by the other guests at his parties.  Hence, we are given only rumors and the clues provided by Gatsby himself.

At this point in the novel, though, Gatsby is about to meet Daisy's husband Tom for the first time.  A few paragraphs later, Tom and Daisy attend one of Gatsby's parties. Tom is a true blue-blooded snob, and Gatsby's rival.  The reader needs to know about Gatsby's lower-class background in order to appreciate how insecure he is around Tom.  Otherwise, we might not be able to sympathize with Gatsby and might lose interest in him, which would be fatal to the novel.  The tension between Tom, Daisy, and Gatsby, is going to keep building until the end of the book, and for the reader to appreciate and feel this tension, we have to understand Gatsby.