Combine the cranberries, ½ cup water, and the ginger in a heavy-bottomed medium-sized saucepan. Cook over medium-low heat until the berries finish popping and release their juices, 8 to 10 minutes. Remove from the heat, and let cool slightly. Puree and strain.
Pour 2 tablespoons cold water into a microwave-safe bowl. Sprinkle the gelatin on top. Set aside.
Combine the sugar and the remaining ¼ cup water in a medium-size saucepan. Bring to a boil, and cook the syrup until the sugar is dissolved. Stir in the ginger liqueur if using. Add the cranberry puree to the sugar syrup, and stir to combine completely.
Heat the gelatin mixture in the microwave for about 10 seconds, or just until the gelatin dissolves completely. Stir it into the cranberry mixture. Remove the panna cottas from the fridge, and pour a thin layer of the cranberry mixture on top, dividing it evenly between the ramekins. Refrigerate until set, about 4 hours or overnight.