Can't clean gunk off frets

So I recently traded for a 2001 Gibson Les Paul Special. Got some dings, but I love it. Finally took the strings off, taped everything up, and I'm giving it a good clean. However... no matter how hard I scrub those frets, I still have black gunky residue coming off on the rag. I've gone over them carefully with steel wool, a little alcohol, and then hit them with the polish—and every single time—gunk. I've got some naptha coming tomorrow, so maybe that will work much better?

Is this a case of 25 years of stubborn gunk? Or am I doing something wrong? Thanks in advance for the help!