Make the Strawberry Syrup: In a bowl, add the strawberries and the syrup. Let the mixture sit overnight. Strain the syrup into a lidded container and discard the strawberries. Refrigerate until ready to use.
In a blender, add the gin, Campari, lime juice and 5 ounces of the strawberry syrup. Fill to the top of the blender with ice and blend until the mixture is the texture of a slushie. Pour into a pint glass, garnish with the basil leaf and serve.