How to Make It
Combine 12 oz. seedless black grapes (about 2 cups), water, sugar, and lemon rind strip in a small saucepan over medium-high. Bring to a boil; reduce heat, and simmer 14 minutes or until liquid is slightly syrupy, stirring occasionally. Cool completely. Discard rind.
Place grape mixture in a blender; pulse until smooth. Strain through a fine sieve over a bowl, pressing gently to extract liquid; discard solids.
To prepare each cocktail, fill each of 4 (8-oz.) glasses with ice and, if desired, frozen grapes. Pour 1/3 cup grape mixture into each glass. Add 1 oz. gin and 4 oz. kombucha. Garnish, if desired.