Rosemary simple syrup: Combine ingredients in a small pot and heat gently to a simmer, stirring, until sugar has completely melted. Leave to cool, then discard rosemary. Leftover syrup can be kept in the fridge for up to a week.
For the cocktail: Muddle grapes in the bottom of a cocktail shaker. Fill the cocktail shaker with ice and then add remaining ingredients. Shake well and fine strain into a large wine glass. Add ice, then top with soda water, to taste. Garnish with a rosemary sprig, if desired.