In a saucepan over medium-high heat, cook the cranberries, 2 cups cranberry juice, and sugar until the mixture is bubbling and the cranberries have burst. Meanwhile, sprinkle the gelatin over the remaining 1/4 cup cranberry juice to soften. Whisk the gelatin mixture into the hot cranberries. Strain this into a bowl and let cool. Spray your mold with nonstick cooking spray and pour the cooled juice into the mold. Refrigerate for at least 3 hours or overnight. To unmold, briefly dip the mold into hot water to loosen it.