Make the juice. Cut the watermelon, scoop out the flesh, blend it, and strain until you have 350 g (1½ cups) of juice. Straining is optional: the texture comes out smoother, but if you'd rather keep the pulp, skip it.
Bloom the gelatin in the juice itself. Set aside 100 g of the juice (cold works best) and sprinkle the 20 g of gelatin over the top in a fine, even layer across the whole surface. Don't dump it in one spot. Stir lightly and let it sit 5 to 10 minutes, until it looks like a thick paste.
Heat without boiling. Warm the remaining 250 g of juice in a saucepan or the microwave until very hot, but never boiling: boiling weakens the gelatin and the gummies won't set properly.
Combine. Add the bloomed gelatin to the hot juice and stir well until it's completely dissolved.
Molds and fridge. Fill your molds and refrigerate for at least 6 hours. No molds? Use a container and cut into cubes afterward.
Unmold and enjoy. If they resist coming out, pop the molds in the freezer for 10 to 15 minutes right before unmolding: they release cleanly and with a nice shine. (Freezer for this step only, not for storing them.)
