Place a crumpled sheet of aluminum foil in an 18 x 28-cm (7 x 11-inch) dish. Set aside.
Slice the oranges in half horizontally. Using a citrus press or juicer, gently juice the oranges without damaging the peels. You will need 375 ml (1 1/2 cups) juice. Top up with store-bought orange juice as needed. With a spoon or melon baller, carefully remove the pulp. Arrange the 8 hollowed orange halves on the foil, taking care to make them stable and level.
In a saucepan, sprinkle the gelatin over 250 ml (1 cup) juice and let soften for 5 minutes. Add the sugar and warm the mixture over low heat, stirring constantly, until the gelatin and sugar dissolve. Add the remaining juice and mix well.
Pour the juice mixture into the orange peels, filling them completely. Refrigerate for 8 hours or overnight.
Using a moistened knife with a thin, very sharp blade, slice each orange half into sections.