In a small bowl, sprinkle gelatin over 1/4 cup grape juice; let soften 2 to 3 minutes.
Meanwhile, heat sugar with another 1/4 cup grape juice in a small saucepan over medium-high heat until dissolved. Remove from heat; stir in softened gelatin until dissolved, then stir in remaining 1 1/2 cups grape juice.
Place berries in a 4-by-8-inch (6-cup-capacity) loaf pan; pour gelatin mixture over, pressing berries gently to submerge completely (remove a few berries if necessary). Refrigerate until firm, at least 3 hours.
To unmold, dip bottom of pan in hotwater about 5 seconds. Invert onto a serving platter, and shake firmly to release. Slice to serve.