Take your torch into a local welding supply shop. They have cleaning wire things for torches with multiple sized wires for a few bucks. They can show you how to do it if you don't know how. For a GTT, I'd only use the ones they provide. If those won't clean it, send it in to them.
Deep cleaning on a torch is something I know nothing about. I'm assuming they tear down the torch and put it back together, but that's not something I'd reccomend doing yourself.
