The worst gelato is still really good. Gelato stores are all over the city and Italians eat it at all times of the year. If it's in a plastic or cardboard container, then they probably don't make it themselves. Second, check the banana flavor. If it's a bright yellow they make it out of artificial colors and flavors.
Natural banana is a grey color.
Pass on the store if either of these is true. The guys behind the counter are rude bastards at times, but it is worth it, believe me. It's north and a little east of the Pantheon. Ask a local, they'll tell you where it is. People Interaction Speaking a little Italian goes a long way with the people.