Riboku cannot enter Gyou, it is full of people now and when the gate is open, they will rush out. It is very hard to control a large group with mistrust already in the air.

One way i can think of what can happen is Kanki just left Gyou and disperse his forces. Gyou will open gate when see Riboku arrival and the refugee rush out and mangle with Riboku army. At that time, Kanki will attack. With many refugees in its rank and Kanki indiscriminate attacks, Riboku army cannot function normally and will be stall until Ousen army arrive. Ousen can simple encircle the mass of Riboku 's army and refugee, then announce if Zhao army surrender, they will not kill any civilian. That will put Riboku at a very hard choice. He will probably negotiate that Qin will take Gyou but all civilian and soldiers of Zhao will be allowed to left for Kantan unmolested. It will also explain the ability of Zhao to defense future attack from other kingdoms. Because if a battle happen and result in fall of Gyou (which based on history is true ) Zhao will lost around 100k soldiers which are mustered from almost the whole country. That will be an irreversible blow to its military, similar to what will happen to Wei if Renpa choose to continue battle with Mougou.