There usually is a cart located at the Italy Pavilion at Epcot that sells 3 or 4 different flavors of gelato. It usually is on the east side of the pavilion going toward Germany. Tutto Italia and Via Napoli have it as a dessert on their menus.
I'm sure several other restaurants not in Epcot do also, such as Il Mulino at the Swan and Portobello in Downtown Disney. Not sure about the other WDW Parks.