How to Make It
Stir together salt and zest on a small plate using your fingers until evenly combined. Line the rim of a highball glass with water; dip rim in salt mixture. Set aside.
Place grapefruit juice, gin, and ice in a cocktail shaker; cover and shake until thoroughly chilled, about 10 seconds. Strain into prepared highball glass; garnish with grapefruit slice. Serve immediately.
Southern Living Test Kitchen used Beefeater gin for testing.