In the bottom of a shaker, muddle simple syrup, grapefruit juice, and basil together. Add gin and coconut milk. Fill with ice. Shake for 25-30 seconds, then strain into a chilled cocktail glass.
To make honey simple syrup:
Add water and honey to a small sauce pan over medium heat. Stir gently until honey has dissolved completely, then remove from heat. Transfer to a storage jar and allow to cool completely before use.