Nick tells us his disillusionment with mankind. In your opinion, is it Gatsby or something else that causes this disillusionment?

His disillusionment is with those that seem to embody the
fulfillment of the American Dream and who feel no responsibility to their fellow man.
 Those, like the Buchanans, who are "careless people"
who

smashed up
things and creatures and then retreated back into their money or their vast carelessness
or whatever it was that kept them together, and let other people clean up the mess they
had made . . .

are the ones
who earned Nick's contempt.  At the end of the novel, Nick looks across the bay and
tries to see it as those first settlers saw New York.  It was a land
that

pandered
in whispers to the last and greatest of all human
dreams.

But the realization
of those dreams in the form of the Buchanans results in empty and careless lives.  Daisy
wonders aloud what do people plan, and Tom attempts to find meaning in polo horses and
mistresses.  The result of the work of the early settlers is excess, stagnation,
boredom, and selfishness.

Nick is not disillusioned with
Gatsby because Gatsby was still reaching for something beyond him--no matter how
unattainable.

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Gatsby believed in the green light, the orgastic
future that year by year recedes before
us.

Gatsby had hope that the
future would be better.  Like Mrytle, Gatsby was striving for more than the status quo.
 The others remained stagnant, entombed in their vast wealth.