Chill a coupe glass in the freezer for 10 minutes or fill with ice water while preparing the drink.
In a cocktail shaker filled with ice, combine gin, fresh grapefruit juice, and lavender simple syrup.
Shake vigorously for 10-15 seconds until well chilled.
Double-strain into the chilled coupe glass.
Express grapefruit peel over the surface of the cocktail to release oils, then drop into the glass.
Garnish with a fresh lavender sprig if desired. Serve immediately.