it's probably carbon & build-up. it'll happen after even one time of shooting. it's easier to coat it in some kind of solvent ( i use hoppe's 9, there's lots of types ) & let it sit for a few minutes, maybe while you're cleaning something else, then scrape it with something. I use an old brass pin punch, works pretty well, or a tiny flathead screwdriver. make sure the rest of the bolt is clean of course, including inside.
i haven't seen the outside of the gas tube get carbon buildup before, just a little discoloration. normally for me a little CLP rub ( with a q-tip ) will get any gunk off, but i read somewhere (sorry i can't remember) that you shouldn't try to scrape off the discoloration. the fitting is really close, it might have to do with the slight bulge near the tip & not damaging it. cleaning the inside of the key is necessary though, where the gas tube hits the carrier, & can be done with a q-tip & cleaner.
keep up your maintenance pretty regularly (i'd suggest after every shooting) & it'll be easier. it'll also keep you from having any kind of malfunction or problems for a long time. always have a good supply of CLP & a better cleaner, like the hoppe's 9. happy shooting