Ala Dan's method will also work. The key is that it's going to take more than just solvent. Either a steel brush, or some sort of abrasive polish.
For the record, I'm becoming less anal these days about cleaning, but I've put a few thousand rounds through one of my GP100s and cleaned the cylinder front after each range session until it's indistinguishable from new condition. The barrel cylinder gap is still comparable to a new GP100 so if I've worn anything off, it's not measurable.
If you like cleaning or really HAVE to get it clean then you now have a couple of ways to get it really clean. I still recommend that you just get the crud off with a bronze brush and some solvent and then leave the rest. It's not hurting anything unless you get careless and let carbon or lead build up