In a cocktail shaker, add fresh watermelon and basil leaves.
Muddle gently to release the flavors.
Add gin, basil syrup, and fresh lemon juice to the shaker.
Fill the shaker with ice cubes and shake vigorously for about 15-20 seconds.
Strain the mixture into a Collins glass filled with ice cubes.
Garnish with a basil sprig or a small wedge of watermelon.