In a medium saucepan, combine 1 cup of granulated sugar, ½ cup of water, and the zest of one grapefruit. Bring the mixture to a gentle boil over medium heat, stirring continuously until the sugar is completely dissolved. This should take about 3-5 minutes.

Once the sugar mixture is ready, remove it from the heat and let it cool completely at room temperature for approximately 30 minutes.

In a large mixing bowl, combine the cooled syrup with 2 cups of freshly squeezed ruby red grapefruit juice and ¼ cup of Campari liqueur. Stir well until all ingredients are fully integrated. Cover the bowl and refrigerate the mixture for at least 1 hour.

After chilling, pour the mixed ingredients into an ice cream machine. Churn until it achieves a soft-serve consistency, usually around 20-25 minutes.

Transfer the churned sorbet into a shallow, airtight container, smoothing the top with a spatula. Freeze for 2-3 hours or preferably overnight to achieve a firm texture.