What could nick mean when he concludes this has been a story of the west after all?

I think Nick means to contrast the decadence of the East with the real world of the West. Both he and Gatsby are midwesterners. They go to the East for different reasons but the East seems to corrupt and morally ravage all who enter. THe lifestyle of decadence and selfishness destroyed Gatsby and Nick is going to run back to the West, back to a place that is real- A. C. S.