Peel wide strips of zest (avoiding the bitter white pith underneath) from half the grapefruit and the whole lime. Juice grapefruit and lime into a liquid measuring cup (add water to reach 1 cup if needed). Pour into a small saucepan and add zest strips.
Cut off green tops from lemongrass (about half the stalk) and peel first two outer layers from stalks. Mash stalks with a meat mallet and cut into 2-in. pieces. Add to pan of juices along with sugar and honey.
Heat citrus lemongrass mixture over medium heat until boiling; boil 1 minute, then remove from heat and let cool. Chill overnight, then strain syrup into a glass jar or measuring cup, discarding solids.
To serve, fill an 8-oz. glass halfway with ice cubes, add 2 tbsp. syrup (or more if you like), top with carbonated water, and stir.
Note: Nutritional analysis is per serving.