Pulse the cranberries and 1/4 cup sugar in a food processor until chunky and juicy, about 8 times. Set aside for 20 minutes to macerate.
Meanwhile, combine the remaining 1/2 cup sugar, 1 1/2 cups pomegranate juice, the cinnamon stick, peppercorns, orange peel and a pinch of salt in a saucepan over medium heat; simmer, stirring, until the sugar dissolves. Remove from the heat; let steep for 20 minutes. Discard the cinnamon, peppercorns and orange peel.
Place the remaining 1/2 cup pomegranate juice and 1/4 cup cold water in a bowl and sprinkle the gelatin on top. Set aside without stirring until the gelatin softens, about 5 minutes. Add the steeped pomegranate juice to the gelatin mixture and stir until the gelatin dissolves.
Spread a thin layer of the cranberry mixture in a 9-by-5-inch loaf pan; pour in enough of the pomegranate-gelatin mixture to cover (about 3/4 cup). Refrigerate until just beginning to set, about 30 minutes. Meanwhile, stir the remaining cranberries into the remaining gelatin mixture and set aside, unrefrigerated. Once the bottom layer is set, pour in the remaining gelatin mixture and refrigerate until completely set, at least 2 hours or overnight.
To serve, run a knife around the edges of the pan and invert the terrine onto a platter. (Set the pan in warm water to soften the gelatin if necessary.) Garnish with the orange zest.