John Galt is Prometheus who changed his mind. After centuries of being torn by vultures in payment for having brought to men the fire of the gods, he broke his chains—and he withdrew his fire—until the day when men withdraw their vultures.
Francisco says this to Dagny in Part Two, Chapter V, after they discover the words “Who is John Galt?” scratched into a table at a restaurant. She says there are so many stories about him, and Francisco tells her that all the stories are true. Metaphorically speaking, they are, and Francisco’s Prometheus story is especially apt. Prometheus was a figure from Greek mythology. He was a titan who stole fire from the gods and brought it to men to improve their lives. In return, he was chained to a rock and tortured. Vultures ate his liver each day, only to have it grow back at night to be eaten again. In Francisco’s comment, Prometheus (personified by Galt) represents the great industrialists who have provided men with prosperity and improved their lives with their inventions and products, but have received only condemnation and government interference in return. These men, led by Galt, have disappeared and taken their prosperity-generating minds (the “fire” they had provided) with them. They will no longer allow themselves to receive torture as payment for their talents, and they will only return their talents to the world when they are no longer punished for bringing them.