Trimalchio is a character from the ancient work The Satyricon, known for his work ethic, his great wealth, and his habits of throwing elaborate parties for his friends with strange and exotic foods. Jay Gatsby is compared to Trimalchio once, as his Thursday night parties end:
It was when curiosity about Gatsby was at its highest that the lights in his house failed to go on one Saturday night -- and, as obscurely as it had begun, his career as Trimalchio was over. Only gradually did I become aware that the automobiles which turned expectantly into his drive stayed for just a minute and then drove sulkily away.
(Fitzgerald, The Great Gatsby, mrbye.com)
Nick, the narrator, supposes that the entire reason for the parties was to meet and court Daisy, and now that she visits regularly, the parties have no more purpose. He also wonders if Daisy disapproved of the lavish and decadent parties, prompting Gatsby to stop them for her sake. The comparison by Fitzgerald is deliberate; one working title for the novel was Trimalchio, and in fact an early draft of the novel was published under that title. Trimalchio's excesses mimic Gatsby's, although Trimalchio worked entirely for his own amusement while Gatsby had a specific goal in mind. The main comparison is the lavish extent of the parties that Gatsby hosts, and his reputation among the guests, who don't care about his hidden purposes and only want to be seen in his presence; when the parties stop, the guests are "sulky" because they don't think of Gatsby as a person but as a facilitator for partying.