A site on the web I feel I can trust has "it was called the Hercules Gate due to having a relief of Hercules on it. It was brought from another place in the fourth century AD to its current place, but the relief on it dates back to the second century AD. Only the two sides of the columns remain today and the other parts of it have not been found. The relief of the flying Nike in the Domitian Square is thought to also be a part of this gate. The Heracles Gate narrowed the access to the street, preventing the passage of vehicles. We can understand that from the Fourth Century, the street had become a pedestrian area."