The description given in wikipedia for this painting is as follows:
A depiction of Phryne, a famous hetaera (courtesan) of Ancient Greece, being disrobed before the Areopagus. Phryne was on trial for profaning the Eleusinian Mysteries, and is said to have been disrobed by Hypereides, who was defending her, when it appeared the verdict would be unfavourable. The sight of her nude body apparently so moved the judges that they acquitted her. Some authorities claim that this story is a later invention.
We can find further details about this on another wikipage about Phryne:
When accused of profaning the Eleusinian Mysteries, she was defended by the orator Hypereides, one of her lovers. The speech for the prosecution was written by Anaximenes of Lampsacus according to Diodorus Periegetes.  When it seemed as if the verdict would be unfavourable, Hypereides tore open her robe and displayed her body, most notably her breasts, which so moved them that they acquitted her. According to others, Phryne herself removed her own clothing. The judges' change of heart was not simply because they were overcome by the beauty of her naked body, but because such unusual physical beauty was often seen as a facet of divinity or a mark of divine favor during those times. Most works of art depicting the trial show Phryne fully nude, not just bare-breasted.
This is then the story/history/legend of Phryne who was famously beautiful, how then can we explain this painting by Gerome? Judging from the shock, perplexion, horror, anger, on the faces of the various spectators in Gerome's painting we might imagine that a monster from outer space has just been revealed before them... and certainly not the most beautiful female on the planet! Note: there are plenty of reproductions of this painting on the internet most are over exposed which then reduces the already minimum detail of Phrynes body. Here too we have to ask if this is Gerome's idea of the epitome of feminine beauty? There are number of very odd/mysterious things about this painting. Gerome was a master of the art of detail, painting photorealistic spectacles were his specialty, why then are so many details amiss in this painting? The geometric pattern on the floor for example, showing an impossible symmetry?