Stir the grapefruit juice, liqueur and simple syrup together in a large bowl. Transfer to a 9-by-13-inch baking dish and freeze. Scrape the mixture with a fork every hour or so until it is icy and spoonable, about 4 hours total. (Note: After the first scraping or two, you can freeze the granita overnight; just scrape again before serving.)
Spoon the granita into serving bowls. Top with a scoop of vanilla gelato and some more granita. Serve immediately.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil over medium heat. Boil until the sugar is dissolved. Let cool, then transfer to a container. Cover and refrigerate for up to 1 month.