The Japanese chili oil that can season a gyoza tare sauce is rāyu. Also written rayu or rāyu, it is a Japanese-style chili oil commonly mixed into the soy-based dipping sauce served with gyōza.
A typical gyōza tare combines soy sauce with rice vinegar; rāyu adds heat, fragrance, and a reddish sheen. Some versions contain toasted sesame oil, garlic, or suspended chili flakes. Diners often add it themselves, so the amount can range from a few drops to a much hotter spoonful.
Rāyu is not the same thing as miso, mirin, or ponzu. Miso is fermented soybean paste, mirin is a sweet rice seasoning wine, and ponzu is a citrusy soy-based condiment. They may appear in Japanese cooking or dipping sauces, but they do not specifically name the chili oil.
Gyōza themselves are the Japanese adaptation of Chinese jiaozi. Japanese-style gyōza are especially associated with pan-frying, and their sauce is a familiar part of ramen-shop and home cooking culture.