Place sugar in a large pitcher or mason jar. Add grapefruit peel and muddle together to release oils from peel. Add grapefruit segments and continue to muddle. Allow mixture to sit for 10 minutes. Stir in vinegar. Cover and let steep in the fridge for 2-4 days, depending on desired flavor profile. Strain out solids. Store syrup in the fridge until ready to use.
Add 1 oz grapefruit shrub syrup to a serving glass filled with ice. Top with chilled sparkling water. Stir to combine. Garnish with fresh thyme, if desired. Enjoy immediately.