“Gun” isn’t one single tech; you start with a canon that’s hard to move, dangerous to the user, and with a smooth barrel and ball that doesn’t have good accuracy.

Industry to make gunpowder cheaply and reliably rather than as one-offs for anti-castle siege weapons, and metallurgy and tooling to make the weapons hand-held and not prone to exploding in the operator’s hand, are what I have to assume old Asia lacked.