What is the effect of the reader's getting the information out of order?
We don’t know the truth about Gatsby until Chapter Six, and we don’t know the rest of the truth until Chapter Eight. We get even more information when Jay’s father shows up. What’s the deal with that?
The creation of Jay Gatsby is what the novel is all about. James Gatz creates this alter-ego, Gatsby, in order to actually become someone else. He goes from rags to riches in search of the American Dream, which, for him, is Daisy and perhaps at certain points, material wealth. We know this because of James' relationship with Dan Cody. Gatsby returns from the army and gets involved with drug runners and gamblers only in order to sustain his wealthy position so that he may at least potentially be in similar social circles as Daisy. No one character in the story knew of his true identity until after Gatsby's death.
Fitzgerald could have waited until the end of the novel to let the readers in on the truth of James Gatz. He gave us some information in the chapters you mention to at least make the reader aware of one of the novel's central themes which is appearance versus reality. Not sharing the truth about Gatz until Chapter 6 duped the reader, as it did the other characters, into believing that Jay Gatsby was a real person. The truth comes as more of a shock because it comes later in the novel. This underscores the fine but certain line between apparent truth and actual truth. It also parallels the superficiality of the socialites who frequent Gatsby's parties. It was tragically fitting then, at the end, when none of them attended his funeral, quite simply because they really didn't know him. They used him (his alter-ego) for the parties, the notoriety. Everyone, reader included, bought in to idea that Jay Gatsby was real/true.