Place ½ cup water in a small bowl; sprinkle with the gelatin and let sit for 5 minutes. Meanwhile, in a medium saucepan, combine the cranberries, sugar, ginger, and ½ cup water. Bring to a boil, reduce heat, and simmer, stirring often, until the cranberries begin to burst, 3 to 5 minutes.
Transfer the cranberry mixture to a blender and puree until smooth. Pass the mixture through a fine-mesh sieve into a medium bowl. (Discard the solids; you should have about 1½ cups of liquid.) Whisk in the gelatin mixture and transfer to a 2-cup mold. Refrigerate until set, at least 4 hours.
To unmold, dip the mold in a bowl of hot water for 10 to 15 seconds, then turn out onto a plate.