01 - Zest one grapefruit and set the zest aside.
02 - Juice the grapefruits to yield about 1.5 cups juice. Strain to remove pulp and seeds.
03 - In a small saucepan, combine sugar and water. Bring to a simmer over medium heat, stirring until sugar dissolves completely. Remove from heat and let cool to room temperature.
04 - In a large bowl, mix together the grapefruit juice, zest, lemon juice, salt, and cooled simple syrup.
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 manufacturer's instructions, about 20–25 minutes, until thick and slushy.
07 - Transfer the sorbet to a freezer-safe container. Cover and freeze for at least 4 hours or until firm.
08 - Let sit at room temperature for 5–10 minutes before scooping and serving.