Hades (the name of John's hometown): Hades was the Greek God of the Underworld; the Underworld itself can also be referred to as "Hades."
Midas (the name of John's prep school): King Midas was a mythological Greek figure who turned all that he touched into gold – eventually to his own detriment.
The Twelve Disciples (the "men of Fish"): This is an indirect allusion to the Twelve Disciples of Jesus in the New Testament.
El Dorado (6.24): The Washington estate is compared to this hidden, mythological city of great wealth.
Dante Alighieri,, The Inferno (1.8): This one is an indirect allusion. John's father muses on the "old-fashioned Victorian motto" that stands "over the gates of the town." He finds it to be "a little depressing." This alludes to the inscription which, in Dante's Inferno, stands over the gates of Hell: "Abandon all hope, ye who enter here."
William Shakespeare, A Midsummer Night's Dream (2.30): Jasmine initially appears to John as a sort of Titania, referencing the character in Shakespeare's play (FYI: Titania was the fairy queen).
Aeschylus, Prometheus Bound (10.13): Braddock is described as "Prometheus Enriched" as a twist on the title of this Ancient Greek play.
Voltaire, Candide (Watch us argue in "What's Up with the Ending?" that Fitzgerald alludes to Candide's great satire. Of course, we could be wrong.)
King Croesus (2.26): Croesus was the King of Lydia in 560 B.C. He was famous for his excessive wealth.