01 - Combine sugar and water in a small saucepan. Heat over medium heat, stirring constantly until sugar completely dissolves. Remove from heat and allow syrup to cool to room temperature.
02 - Whisk cooled simple syrup with grapefruit juice, zest, and lemon juice in a large bowl until thoroughly incorporated.
03 - Transfer mixture to ice cream maker and churn according to manufacturer's instructions until soft-serve consistency forms, approximately 20-25 minutes.
04 - Pour churned sorbet into freezer-safe container, cover tightly, and freeze for 4-6 hours until firm.
05 - Scoop sorbet into chilled bowls or glasses. Top each serving with 2 fresh basil leaves and serve immediately.