If your essay is analytical (and I'm struggling to think of any other reason you'd write an essay about The Great Gatsby) then I'd put it in the present tense.
Gatsby loves Daisy, but Daisy is married to Tom. Gatsby doesn't have the bloodline to impress her; all he has is money. So he throws lavish affairs at his ostentatious house in a effort to show her how riche he is, and only comes off looking painfully nouveau.
Even though the book is in past tense, as you read it you are in the book's "present," so you are in the action. When you're analyzing it, you're analyzing what occurs. You'd use past tense if you were talking about something which happened in the character's past:
Jane Eyre is hired by Mr. Rochester as a governness. When he asks her if she can play the piano, she modestly replies, "A little," and proceeds to reel out some Chopin. She learned the piece when she was a child, when she lived at Lowood. Mr. Rochester snorts at how English girls are brought up to downplay their achievements.