JOIN USAussie Melissa wins a thriller
Gorman beat double world champion Larisa Ilchenko after the Russian missed touching the finish with her final stroke and had to settle for silver.
Officials took several minutes reviewing television footage before Gorman, who had led for most of the race, was declared the winner.
“She’s been my idol and to beat her is amazing. I can’t believe I actually got there,” Gorman told reporters.
Brazil’s Poliana Okimoto took bronze under slightly murky skies at the beach resort close to Rome.
There was none of the physical contact which has marred previous open water races although Olympic 10km champion Ilchenko and Gorman swam next to each other in a thrilling final straight.
Better late than never
Antigua's open water swimmer Kareem Valentine finished over half an hour behind the world champion in Tuesday's 5km ocean race -- but at least he made it just in time for the medal ceremony.
Valentine was so far behind the rest of the field that organisers were already preparing to give Thomas Lurz his gold medal when the straggler clambered ashore to wild applause. Most spectators on the beach had not realised he was still in the water and officials could not give him a time because he was over their limit.