what Gatsby`s death ultimately teach nick?

The book is actually pretty dark. Nick sees two worlds. Gatsby's world is corrupt largely hidden, overlaid by a social veneer of wealth, glamour and beauty. His mansion, lawns, beach, motor cars, speed boats, clothing, and expensive household furnishings work together to create the illusion of social standing. Gatsby's parties, played out like theater productions, are memorable for their excess.Glamour went hand in glove with celebrity, and corruption in all forms- moral, ethical, political, and economic- made for great copy and great celebrity. Gatsby's world represents this with Jordan, Tom, and Daisy who are glamorous in their East Egg society, but morally and spiritually corrupt and bankrupt.

I think, in the end, Nick is left disillusioned with the disease that is money. The relentless obsession and pursuit of wealth is the American dream and it is rotten.