Place the cranberries in a saucepan, add 50 ml of water and place the saucepan on low heat. Cook for 7 minutes after boiling.
Remove the cranberries from the heat and grind them using a blender, then strain through a fine sieve. Place the resulting juice over medium heat, add sugar and stir until completely dissolved. Cook the juice, stirring, for about 10 minutes.
Remove the juice from the heat, add the gelatin and stir until completely dissolved.
Line the pan with parchment paper, pour the juice into it and refrigerate for 5 hours.
Slice the marmalade and serve.