Combine water and sugar in a medium saucepan. Bring to a boil, reduce heat to low, and simmer until sugar is dissolved, about 3 minutes. Stir in lemon juice.
Add the marshmallow cream and whisk briskly over very low heat until the mixture is creamy and smooth, approximately 4 minutes. Whisk in the sour cream. Pour the mixture through a strainer into a medium-sized bowl. Cover tightly with plastic wrap and refrigerate until cool, approximately 1 hour.
Remove the mixture from the refrigerator. Whisk if separated and then pour into the freezer compartment of an ice cream maker. Freeze according to manufacturer’s directions. (If you don’t have an ice cream maker, pour the gelato mixture into a shallow baking dish and freeze until solid, approximately 6 hours. Break into chunks and pulse in a food processor until smooth.)
Serve immediately or transfer to a lidded plastic container and freeze.
Nutritional information per serving (1/2 cup): 330 calories, percent fat, 98 percent carbohydrate, 2 percent protein, 31 milligrams calcium, negligible iron, 3 micrograms folic acid.