Cai Jing
It is not known when Gu Yanlang was born. According to his biography in the "New Book of Tang", he was from Feng Prefecture (豐州, in modern Bayan Nur, Inner Mongolia). Both he and his younger brother Gu Yanhui served as junior officers at Tiande (天德, in modern Bayan Nur). The defender of Tiande, Cai Jing (蔡京), considered both of them to have appearances that indicated that they would be later successful, and therefore had his son give them money. During the subsequent major agrarian rebellion led by Huang Chao, Gu Yanlang and Gu Yanhui served in the campaign against Huang and participated in Tang's recapture of the capital Chang'an from Huang. Gu Yanlang was subsequently made a general of the imperial guards.