How to addforce to the player in opposite direction of gun rotation?

Hi, I wanted to make it so that when the player fires the gun, which rotates depending on the mouse position, will add a force to the player sending them in the opposite direction, sort of like a knockback effect.

I know I have to use something like rb.AddForce(); to the player however I am unable to figure out how to use the gun's rotation as a way to depend what force is added. The gun is a child of the player GameObject.

Many thanks!