San Gio was the home of the pope for 13 centuries before the Vatican was built. This is the location where Luther would have come first to Rome to see the pope; here is the site of the famous steps some Catholics climb on their knees. And which are still covered with walnut shells.
San Gio palace has been cut to make way for a road. But the church is still there along with the famous pine come door (which will help you get pregnant of you touch one of the pine comes) and a massive statue of Constantine.