Pour 3 tablespoons water into a small bowl and sprinkle the gelatin on top. Let the gelatin soften for a few minutes. Meanwhile, heat the chicken stock in a medium saucepan until hot but not boiling. Remove from heat and add the gelatin mixture. Whisk until combined. Pour the liquid into a 13x9x2-inch baking dish. Cover and refrigerate until solid. Once solid, cut the block into long strips and then cut the strips into cubes, about ½-inch in size.
Mix the chicken, shrimp, scallions, ginger, garlic, soy sauce, wine, sugar, sesame oil, salt, and pepper together in a bowl.
To make the dipping sauce, mix the soy sauce, vinegar and ginger together in a small bowl. Refrigerate until ready to use.
To make the dumplings, place one dumpling wrapper on a cutting board or flat surface and place a teaspoon of the filling in the center. Add 2 or 3 cubes of gelatinized broth to the filling, pressing them in. Lift up one edge of the dumpling wrapper and make small pleats or folds in the dough, working your way around the entire circle until the dumpling is closed. Pinch the top to seal it. Assemble the remaining dumplings.
To steam the dumplings, pour water into a steamer to a depth of 1 or 2 inches. Line the steamer basket with a few cabbage leaves and spray it with cooking spray. Place several dumplings on the cabbage leaves, leaving a little space between them so they don’t touch. Close the top and steam until the filling is cooked through, about 10-12 minutes. Carefully remove the dumplings from the steamer. Cook the remaining dumplings. Serve dumplings hot with the dipping sauce on the side.