Bring the water and sugar to a boil in a small saucepan. Once boiling, add the lemon zest, basil leaves and salt. Stir, remove from heat, and cover. Steep the basil leaves in the simple syrup for at least 20 minutes.
Meanwhile, add the lemon juice to a 4-cup measuring pitcher. Juice the grapefruits into the pitcher until you’ve measured 3 cups of combined juices.
Strain the simple syrup and discard the basil leaves. Then mix the syrup with the juice. Refrigerate for 2-3 hours (or to speed up, put in the freezer for 1 hour.)
Pour the sorbet mixture into an electric ice cream maker. Turn on and mix for 20+ minutes, until the mixture reaches a “soft serve” consistency.
Eat the sorbet immediately, or put in an airtight container and freeze. After frozen, allow the sorbet to sit out for 10-15 minutes to soften before serving.
NOTE: Make sure to use fresh squeezed grapefruits, not pre-squeezed grapefruit juice, to get the brightest freshest flavor.