Add 1/4 cup of water, 1/4 cup Truvia, and 1-2 Rosemary Springs in a pot on the stove. Bring to a simmer for about 2-3 minutes. Strain into a small container and let completely cool.
In a glass, pour the grapefruit juice and add the Rosemary syrup. Stir well and add ice. Top off with soda water. Use a sprig of rosemary and slice of grapefruit to garnish.