Build drink in a Collins Glass
Start by making the simple syrup. Combine the 3 ingredients in a saucepan and bring to a low simmer.
Stir until sugar is completely dissolved. Continue to simmer for 1 minute.
Remove from heat and allow rosemary to steep for at least 30 minutes.
Remove rosemary sprigs and pour into a sterilized glass jar or bottle. Store in refrigerator for up to 1 month.
Combine Gin, lime juice and simple syrup in a cocktail shaker. Fill shaker with ice, cover, and shake until outside of shaker is cold, about 20 seconds.
Strain cocktail and pour into a collins glass. Top with grapefruit seltzer. Garnish with pinch of flake sea salt and sprig of fresh rosemary. Serve.