Add all ingredients except gin to a large pitcher and stir. Add ice, then top with gin for layered effect. Stir well before serving. Garnish with fresh mint and thyme.
Notes
Watermelon Juice: It takes about 6-7 cups of cubed watermelon to yield 4 cups of watermelon juice. Add the cubed watermelon to a high speed blender, then strain solids from the juice. Compost or dispose of the solids.
Mint and Thyme Simple Syrup: To make, heat 1 1/2 cups of water and 1 1/2 cups of sugar over medium-high heat, stirring until the sugar is dissolved. Remove from heat and add 3-4 sprigs each of mint and thyme. Cover and rest for 10 minutes, then remove the herbs and allow syrup to cool.