Combine the grapefruit zest, grapefruit juice, honey, and ginger in a small saucepan over medium heat. Boil for 2 minutes, stirring to dissolve the honey.
Remove from heat and let cool. Strain the syrup through a sieve into a clean container and discard the solids.
To serve, spoon 2 tbsp of syrup into an 8-ounce glass, top with carbonated water, and stir to combine. Taste and add more syrup, if desired.
The syrup can be refrigerated for up to 1 week.