Combine the orange juice and 115g (1/2 cup) of the sugar in a small saucepan. Stir over medium heat until the sugar dissolves. Bring to the boil, and cook, uncovered, over medium heat for 15 minutes or until reduced to 125ml (1/2 cup). Remove from the heat and set aside.
Use a balloon whisk to whisk the egg yolks, remaining sugar and orange rind in a large bowl until thick and pale. Gradually whisk in the milk until well combined.
Transfer the milk mixture to a medium heavy-based saucepan. Cook over medium-low heat, stirring constantly, for 15-20 minutes or until thick enough to lightly coat the back of a metal spoon. Remove the custard mixture immediately from heat. Transfer to a large bowl and set aside for 10 minutes to cool slightly.
Stir in the orange syrup. Cover the bowl with plastic wrap, and place in the fridge for 3-4 hours or until well chilled.
Stir in the Campari. Pour into a 1.5-litre (6-cup) freezerproof container, cover with lid or foil, and place in the freezer for 4 hours or until frozen.
Remove from the freezer, and use a metal spoon to break gelato into chunks. Transfer to the bowl of a food processor and process until completely smooth, but not melted. Return to the container, cover, and place in the freezer for 3 hours or until frozen.
Transfer the container from the freezer to the fridge for 15 minutes to soften the gelato slightly before serving.
