Combine sugar and water in a large saucepan; bring to a boil, stirring until sugar dissolves. Remove from heat; cool completely.
Combine the pineapple and milk in a large bowl; stir in sugar syrup.
Pour mixture into the freezer can of an ice-cream freezer, and freeze according to manufacturer's instructions. Spoon gelato into a freezer-safe container; cover and freeze 1 hour or until firm.