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