Combine cinnamon stick, cloves, peppercorns, lemon zest, and orange zest in a cheesecloth square and tie it closed to form a spice sachet.

Sprinkle gelatin over 1 tablespoon of white grape juice in a small cup and let it soften for about 1 minute.

In a saucepan, bring white wine, remaining 1 1/2 cups white grape juice, sugar, nutmeg, and the cheesecloth spice bag to a boil.

Remove the saucepan from heat.

Add the softened gelatin mixture to the saucepan, stirring until dissolved.

Transfer the soup to a bowl.

Chill the soup, covered, in a larger bowl of ice and cold water in the refrigerator for at least 8 hours.

Before serving, remove and discard the cheesecloth spice bag.

Add grated lemon zest and sliced grapes to the chilled soup.

Serve chilled.