Prepare an ice-water bath. Whisk whole milk with cornstarch in a bowl.
Whisk together fat-free milk, sugar, and orange zest in a saucepan, and bring to a simmer. Add cornstarch mixture, and cook, whisking, until mixture bubbles and thickens, 2 to 3 minutes. Remove orange zest. Set in ice-water bath to cool.
Puree avocados, lime juice, salt, and milk mixture in a food processor.
Freeze the avocado mixture in an ice cream maker according to the manufacturer's instructions. Transfer to an airtight container, and freeze for at least 3 hours (or up to 2 weeks). Spoon 2 scoops of gelato into each bowl, and serve with lime wedges for squeezing.