Whisk together all ingredients in a small bowl. Taste for heat and add crushed red pepper flakes to increase spice level.
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