Where can the idea of "The Lost Generation" be seen in The Great Gatsby?

I think that part of this is going to lie in your
description or definition of "the Lost Generation."  A part of this is going back to
Stein's definition and clearly establish that Stein uses the term to describe those who
have lost the ability to believe in anything, and have succumbed to a cynical hedonism
as a result.

It is here where some connection to the
flappers in Fitzgerald's work can be present.  Jordan Baker is a great example of
someone who has forgone morality and embraced the pursuit of the self- interested life
as the most important element for her.  Tom Buchanan does not believe in any set of
ideals unless they directly benefit he and his social standing.  Daisy might be able to
believe in some notion of idealism, but she surrenders it so easily for her own
convenience and comfort that one wonders whether it was worthy for her to believe in it
at all.  In this light, one can see that the characters in Fitzgerald's work might be
representatives of a "lost generation" on the basis that they represent much of what the
term means in describing a shift of belief in moral structure to the pursuit of the life
of self- interest.