One gyoza is about 65 calories, which sounds harmless until you count how they are eaten. Nobody orders 2. The plate is 6, the Ohsho double is 12, and a 12 piece order is 780 calories before you have touched your fried rice. The filling itself is not the problem. Minced pork with cabbage is about 35 calories per dumpling. The wrapper adds 15 and the pan oil adds another 15, so half the plate's calories come from the 2 parts you do not taste as food.
The real trap is that gyoza is almost never the main dish. It is the side to a 525 calorie tonkotsu ramen or a 630 calorie plate of chahan. A ramen and gyoza set at a typical shop is 900 to 950 calories, and the beer that usually joins it is another 150. That is a 1,100 calorie lunch for about 1,300 yen, and it does not feel heavy while you eat it because each dumpling is 2 bites.
The dipping sauce is nearly free at about 10 calories for soy, vinegar and rayu chilli oil, unless you pour the rayu with a heavy hand. Rayu is pure oil, 40 calories per teaspoon. Two teaspoons of it in the dish adds 80 calories to a 390 calorie plate. Most people use a few drops. If you use it like a sauce, it is worth knowing.