Whisk together all ingredients in a small bowl. Taste for heat and add crushed red pepper flakes to increase spice level.
While optional, I prefer to allow the gyoza sauce to sit for at least 20 minutes, an hour is preferable.
Notes
I use tamari instead of regular soy sauce to keep the recipe gluten-free. You can use any type of soy sauce that is suitable for your diet including low-sodium soy sauce.
If you don't have chili oil on hand you can substitute ½ teaspoon crushed pepper plus an additional ½ teaspoon sesame oil