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genre: easy listening

This song appears in the novel when Gatsby is trying to impress Daisy in his mansion after they've had tea at Nick's house. Gatsby makes one of his servants, Mr. Kipspringer, play a song on the piano for them while they sit on the couch and listen.
"Jack built long ago
Just a love nest
Cozy with charm
Shall we build for pride         
Warmth and love inside"

Ftzgerald used this song to symbolize Gatsby and Daisy's feelings for one another. Gatsby's mansion could be described as 'just a love nest' because it is where their relationship begins again. Jordan reveals later in the novel that Gatsby chose to live in the mansion to be near Daisy. This could imply that he tried to develop his house to impress Daisy and make it 'cozy with charm.' 

Sounds like an old sweet love tune,
Say that there soon will be a honeymoon."

This song could've been included to symbolize Gatsby and Daisy's reconnection. It shows how their love is rekindling and 'sounds like an old sweet love tune.' The line 'there will soon be a honeymoon' implies the happy ending that Gatsby hopes for with Daisy. This song is a good summary of the connection they share at this point in the novel. It is placed in the story almost at the climax of Gatsby and Daisy's relationship which sets the romantic mood of the story at the time.