In Chapter Eight, Nick makes a judgement call at last. Maybe it was the shock of seeing Tom's malicious hypocrisy and Myrtle's unforgettably violent death on his 30th birthday.
(Hold the phone...Nick's 30??? The clucking Aunts and Uncles who sent him East to chase his fortune, the internship-level job on Wall Street. I thought he was just out of college this whole time.)
maybe it was the slightly-intoxicated, smoke-wreathed all-nighter he
pulled with Gatsby, reminiscing about the past. This is when Nick gets
Gatsby's true history. The one that interrupts the narrative in Chapter Six. It's almost as if Gatsby knows he's going to die.