What is a good quote in the great gatsby, that I can use in an essay?

Try the last line of the book. It is beautiful and well worth an essay.

The Wikipedia article has several links to online copies:

http://en.wikipedia.org/wiki/The_Great_Gatsby#External_links

The Wikiquotes page has a collection of quotes:

http://en.wikiquote.org/wiki/The_Great_Gatsby

last line

from chapter 1:
“Gatsby turned out all right at the end; it is what preyed on Gatsby, what foul dust floated in the wake of his dreams that temporarily closed out my interest in the abortive sorrows and short-winded elations of men.”

from chapter 5:
“One thing’s sure and nothing’s surer. The rich get richer and the poor get – children.”

the last line is very notable and used a lot
from chapter 9:
“So we beat on, boats against the current, borne back ceaselessly into the past.”

They were careless people, Tom and Daisy——they 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. . .
from Chapter 9

Make Google your friend.

Thanks everyone.