How to rotate gun only on X and Y axis?

I have a game where the player is stationary and enemies walk at them. I have a gun in front of the player view that moves left and right / up and down with input from the mouse. However, The Z rotation of the gun always makes itself flip upside down after enough mouse movement.

Here's the code.

 public class RotateGun : MonoBehaviour
 {
     [SerializeField] private float horizontalSpeed = 2.0f;
     [SerializeField] private float verticalSpeed = 2.0f;
 
     // Update is called once per frame
     void Update()
     {
         float h = horizontalSpeed * Input.GetAxis("Mouse X");
         float v = verticalSpeed * Input.GetAxis("Mouse Y");
                 
         transform.Rotate(v, h, 0);        
     }
 }

If anyone can send me in the right direction, I'd appreciate it.