When i change up to down i get an error message sayin that down doesn’t have a definition

When i change up to down i get an error message sayin that down doesn’t have a definition what do I use instead

void FaceMouse()
{
    Gun.transform.up = direction;
}

Try

Gun.transform.rotation = Quaternion.LookRotation( direction , Vector3.up );

Unity considers z+ as forward axis and this will rotate this object to face z+ in provided direction.

If you see strange rotations or warnings it just means that you are about to learn more on somewhat difficult topic of 3D rotations here.