In a large bowl, combine the ground pork, finely chopped cabbage, green onions, garlic and ginger.
Add soy sauce, sake, sesame oil, sugar, salt, and pepper. Mix everything thoroughly until well combined.
Place a gyoza wrapper in the palm of your hand. Spoon about 1 teaspoon of the filling into the center of the wrapper.
Moisten the edges of the wrapper with water using your finger.
Fold the wrapper in half over the filling to create a half-moon shape. Press the edges together to seal, making small pleats along the edge if desired. Repeat with the remaining wrappers and filling.
Heat 1 tablespoon of vegetable oil in a large non-stick skillet over medium-high heat.
Place the gyoza in the skillet, flat side down, in a single layer without crowding.
Add 1/4 cup of water to the skillet and immediately cover with a lid. Let the gyoza steam for about 5 minutes or until the water evaporates.
Remove the lid and cook for another 1-2 minutes until the bottoms are crispy again.
In a small bowl, mix soy sauce, vinegar and chili oil to taste.