In a large bowl, combine the ground pork, shredded Napa cabbage, scallions, grated ginger, and minced garlic. Add the soy sauce, sesame oil, cornstarch, salt, and pepper, then mix with your hands until the filling is well combined and slightly sticky.
Place a gyoza wrapper in the palm of your hand. Lightly moisten the edge with water, spoon about 1 teaspoon of filling into the center, fold the wrapper into a half-moon shape, and pleat the edges to seal. Repeat with the remaining wrappers.
Heat the vegetable oil in a nonstick skillet over medium-high heat. Arrange the gyoza in a single layer and cook for 2–3 minutes, until the bottoms are golden brown and crisp.
Pour in the water and immediately cover the skillet. Reduce heat to medium and steam the gyoza for 5–7 minutes, until the water has evaporated and the wrappers are tender. For extra-crispy bottoms, uncover and increase the heat to high for 1 more minute.
In a small bowl, whisk together the soy sauce and rice vinegar. If desired, add a few drops of sesame oil and chili oil. Serve the gyoza hot with the dipping sauce on the side.