01 - Combine sugar and water in a small saucepan. Bring to a gentle boil, stirring until sugar dissolves completely. Remove from heat.
02 - Add fresh basil leaves to the hot syrup. Cover and let steep for 15 minutes to extract herbal flavor.
03 - Strain syrup through a fine mesh sieve, discarding used basil leaves. Allow syrup to cool completely to room temperature.
04 - In a large bowl, combine pink grapefruit juice, grapefruit zest, cooled basil syrup, lemon juice if using, and pinch of sea salt. Stir thoroughly until well blended.
05 - Refrigerate the mixture for at least 1 hour until thoroughly chilled.
06 - Pour chilled mixture into an ice cream maker and churn according to manufacturer's instructions, typically 20–25 minutes, until thick and slushy.
07 - Transfer sorbet to a freezer-safe container. Cover tightly and freeze for at least 3 hours until firm.
08 - Let sorbet sit at room temperature for 5 minutes before serving to soften slightly. Scoop into bowls and garnish with fresh basil if desired.