Slice a fresh grapefruit into thin wheels. In a small bowl, combine equal parts ground turmeric and ground pink peppercorn.
Lightly dust one side of each grapefruit wheel with the spice mixture just before serving.
Combine the gin, St. Germain, spicy honey syrup, demerara syrup, Supasawa, fresh grapefruit juice, and saline solution in a shaker filled with ice.
Shake vigorously for 10 to 15 seconds until the mixture is well-chilled.
Strain into a chilled coupe glass.
Top with grapefruit soda.
Gently place the dusted grapefruit wheel on the surface of the cocktail, allowing it to float.