It is considered that the Gyotai took in the system of the Tang Dynasty in the early Heian period; however, since there was no dress code in the Yoro-ryo (Yoro Code), there is no document that clearly mentioned when the Gyotai was adopted.
養老令の「衣服令」には規定がなく、平安時代初期に唐代の制度を摂取したものとみられるが、いつ取り入れられたかを明記する資料は無い。