I dunk the end of the gas piston in bore solvent and scrape it off with the pointed tool on the Swiss soldier's knife. Anywhere that sells Swiss Army knives can get them, might have them in stock if you're lucky. It has an aluminum handle instead of the red plastic one.
The pointed tool is also handy for getting the carbon outside of the gas plug. What I do is wrap some paper towel around the end of a rod, wipe the inside around with bore solvent and then scrape off any awkward bits with the pointed tool on the knife.
The steel the gun is made from is harder than the blade steel of the knife, so it shouldn't be a problem. You might scratch the finish inside the gas plug, but who cares.
The real problem I have is that the gas plug sticks to the end of the gas piston, and the manual doesn't really explain how to disassemble it when it gets stuck. I just squirt some CLP on it and work it until it comes loose if that happens.