Put cinnamon stick, cloves, peppercorns, and zest strips on cheesecloth and tie it closed.
Sprinkle gelatin over 1 tablespoon grape juice in a small cup and let soften about 1 minute.
Bring wine, remaining 1 1/2 cups white grape juice, sugar, nutmeg, and cheesecloth bag to a boil in a 1- to 2-quart saucepan, then remove from heat and add gelatin mixture, stirring with a metal spoon (see cooks' note, below) until dissolved.
Transfer soup to a bowl and chill, covered, in a larger bowl of ice and cold water in the refrigerator for at least 8 hours.
Just before serving, discard cheesecloth bag and add grated zest and grapes to soup.
Cooks' notes: · Soup (without grated zest and grapes) can be chilled up to 12 hours. Add zest and grapes just before serving. · A metal spoon is the correct tool for stirring gelatin; a wooden one might transfer oils, and a whisk might incorporate tiny bubbles.