Shake gin, lemon juice and rosemary simple syrup (recipe below) in a cocktail shaker filled with ice. Strain into a glass filled with ice. Add club soda. Garnish with lemon slice and rosemary sprig.Stir gin, lemon juice and the rosemary simple syrup in a pitcher. Refrigerate. When ready to serve, divide between 4 glasses filled with ice, add club soda then garnish with lemon slices and rosemary sprigs.Combine sugar and water in a small saucepan then bring to a boil. Cook, while stirring, until the sugar dissolves.Remove pan from the heat, add rosemary, and then let steep for 1 hour. Discard the rosemary sprig.