Begin making the rosemary simple syrup by heating the sugar and water in a small sauce pot over medium high heat. Bring to a boil then remove from the heat. Whisk until the sugar is completely dissolved. Add the rosemary sprigs then let steep until cool, at least 2 hours or up to three days. Strain the rosemary sprigs before using.
To create a salt rim for the glasses, place about 1/2 cup of coarse Kosher salt on a plate. Run a lime wedge around the rim of four cocktail glasses then gently run the glass rim around the salt to coat; set aside.
In a large pitcher, mix together the gin, grapefruit juice, lime juice, and rosemary simple syrup. Stir well.
Fill each glass with ice then pour the gin and juice mixture 2/3 way full. Top each glass with club soda and garnish with lime wedge or rosemary sprig to serve.
If you don't plan to use the rosemary simple right away, store in an air tight refrigerated container for up to 7 days.