Simple syrup can be purchased, but consider making your own: In a sealable container, mix 1 quart hot water and 750 grams superfine sugar. Cover container and shake vigorously. Transfer to a glass bottle for storage.
To make the cocktail: Measure all ingredients except soda and garnishes in a cocktail shaker. Add a few ice cubes and shake for 5 seconds. Strain into a highball glass with fresh ice, top with soda. Garnish with grapefruit peel and salt.