The novel makes it clear that no amount of money can buy you love - no matter how much of it you may have, it does not ensure true happiness. This fact is brutally and poignantly made clear through the actions of the characters and the unfolding events.
Fitzgerald does not place money and love in opposition to one another, that is, the one does not exclude the other. Instead, he illustrates that even though one may possess an abundance of one, it does not naturally follow that you are also guaranteed the other.
This fact is best illustrated by Jay Gatsby's unrealistic romantic idealism. Jay is desperately in love with Daisy Buchanan, his so-called "holy grail" with whom he almost frantically wishes to re-establish a relationship. He believes that he can achieve this ideal by becoming incredibly wealthy, since Daisy was accustomed to the comfort that wealth brings and had married the impossibly rich Tom Buchanan who is heir to a fortune in "old money" - established wealth.
To a certain extent, Jay succeeds in achieving his dream by reconnecting with Daisy and starting an affair. The dream is shattered, though, when Daisy tells him after his confrontation with Tom that he "wants too much." Daisy was not prepared to sacrifice the comfort and luxury that she had become accustomed to for Jay's love, in spite of the fact that Jay was also enormously wealthy. Although Jay has great love for Daisy, she is too shallow, careless and materialistic to understand the depth of his passion.
We also learn through Myrtle Wilson's affair with Tom Buchanan that wealth, or the desire thereof, does not guarantee happiness. Myrtle was delusional in that she believed that she stood a chance with Tom Buchanan and that she would be happy, but Tom saw her as a mere distraction, a plaything, someone to stroke his ego. He physically abused her and would never leave Daisy for her.
It is tragic that both Jay and Myrtle die in their pursuit of an unrealistic ideal.