I assume that there's no access to the back side of that dent. If that's the case, you're pretty much out of luck. Unless you want to drill some holes on the inside of the gate and push the dent back out. I think it adds to the "character" of the patina.
BTW, I removed your previous nearly duplicate post. Please only post one thread on a subject. You can edit your post if you want to add more info. Thanks
