Add the frozen fruit to a blender and blend until it resembles crushed ice. Transfer to a small saucepan.
Add the allulose and 1 cup of water.
Over medium heat, bring the fruit mixture to a boil, stirring constantly. Make sure the sweetener dissolves completely.
Once the mixture reaches a boil, turn off the heat. Place a fine mesh strainer over a large measuring cup. Pour the fruit mixture into the strainer and use a spoon to stir while pushing it through the strainer.
After straining, you should have 2 cups of liquid in the measuring cup and a jam-like substance left in the strainer. Either discard the jam-like fruit solids, or use them in other recipes or smoothies.
In a separate measuring cup, dissolve the gelatin into the remaining cup of water. Pour this into the liquid in the large measuring cup while stirring continuously.
Pour the mixture into your mold of choice.
Carefully cover and place the mold in the fridge and allow it to set for 3 hours before unmolding.