Which alternate title for the novel mentions a character from Petronius?

The alternate title mentioning a character from Petronius is Trimalchio in West Egg.

Fitzgerald drew the name from Trimalchio, the extravagant and vulgar freedman in Petronius’s Satyricon. The ancient character hosts lavish feasts to display his wealth, making him a useful parallel for Gatsby, whose spectacular parties advertise a carefully manufactured identity and social rise.

Fitzgerald considered several titles, including Trimalchio, Among Ash Heaps and Millionaires, On the Road to West Egg, and The Gold-Hatted Gatsby. By November 1924, he had settled on Trimalchio in West Egg, but editor Maxwell Perkins thought the reference was too obscure and difficult to pronounce. Fitzgerald ultimately accepted The Great Gatsby, which was published on April 10, 1925.

The title is sometimes confused with Trimalchio: An Early Version of The Great Gatsby, the name given to a 2002 publication of an earlier version. That is a later editorial title, not the principal working title asked about here.