Cook 1 1/2 cups cold water and sugar in a saucepan over medium heat, stirring often, until sugar melts.
Sprinkle gelatin over remaining 1/2 cup cold water in a large bowl; stir and let stand 1 minute. Stir in hot sugar mixture until dissolved. Stir in raspberry liqueur.
Pour a 1/2-inch-thick layer of gelatin mixture into an 8- x 4-inch loafpan; chill 30 minutes. Layer raspberries, blueberries, and peach slices over gelatin, pouring remaining gelatin over top of each layer of fruit. Gently shake pan, allowing gelatin to flow into crevices. Cover and chill 8 hours. Unmold onto a serving dish. Slice and serve with Raspberry Sauce. Garnish, if desired.