Gently pour grapefruit juice into ice trays. I like to use a ladle to make this process neat. Freeze until solid, about 8 hours. If you don't have enough ice trays, freeze in batches by placing solid cubes in a freezer storage bag and reusing the trays for the remaining juice.
1 hour before serving, make the simple syrup. Place salt, sugar, and water into a small sauce pan and heat just until boiling, stirring occasionally. Let sit to cool. The sugar will continue to dissolve.
Once the syrup is cooled, it's ready for use. Place ⅓ cup into blender with half of the frozen ice cubes and 2 ounces of gin. Blend until smooth. Taste and add more syrup if necessary.
Pour into glasses and serve with a straw. Blend the rest of the juice ice cubes with the rest of the gin and ⅓ of the syrup.