Looking at the action of this section of the narrative, one of the major themes is truth and the subjectivity of truth.
The "true nature" of a number of characters finds expression here. Tom and Daisy flee after Gatsby and Myrtle are killed, turning their backs on the people who are left behind (Nick in particular) and effectively denying any moral or practical responsibility for what has happened. Daisy, like nearly everyone else, fails or refuses to attend Gatsby's funeral.
These facts add up to a depiction of Tom and Daisy as two people cut from the same cloth, as it were, more alike than they are distinct; equally callous and insulated from the woes of others; equally selfish, weak, and uncaring.
Gatsby is shown in his postmortem state to have been truly isolated. He has no real friends, beyond Nick, and no professional relationships that last beyond his ability to broker a deal or pay real money. In this regard, he is shown as the dreamer that he was/is. His proverbial castle was made of sand. When he is no longer around to maintain the dream, it collapses into nothing.
In the end, Gatsby is only an idea. To his father he is the idea of a son who "made good". This is the truth of his character from one perspective. To Nick, Gatsby is an example of the power of fantasy to persist in the face of reality, of innocence to survive despite a life that should have squashed it completely. Gatsby is proof that goodness can survive even within a world that will not recognize it (and in a person who won't claim it).
The glamour that had characterized many of the characters falls away in this chapter, revealing a more modest (and even squalid truth), yet this truth is not the same for everyone.
Tom admits in this chapter to sending George Wilson to Gatsby's house, claiming that it was an act of self-defense. Wilson would have killed Tom if Tom had not deflected the deranged man. To Tom, this is justification enough. To Nick, this is an indefensible admission of moral weakness and unforgivable selfishness.
Again, the truth is a matter of subjectivity (not to say opinion).
Chapter nine has a mixture of themes that all intertwine in some way. The beginning of the chapter the theme closely relates to the jazz age. It is talked how people were down after the war but started to revive in the "roaring 20's." The optimism that Americans felt shows through with the optimism But the writeer tends to show the hypocrasy and recklessness that many people showed during this time.
Gatsby tries to show how people are valued through money. If you have none, you tend to be just forgotten, unimportant. You have old money, where those people have had it for eons. The old monies tended to be a little stuffy and not flaunt what they had. Then there is new money, these people made their money from from the 1920's boom and loved to flaunt it.