To clean gas system parts like pistons, you can use a "lead-away" cloth made for removing leading and carbon from stainless pistols and hard chromed gas system parts.
(Do NOT use on blued steel, it removes bluing).
A second method is to use a liquid carbon remover like Slip 2000 Carbon Killer.
This is a safe liquid you shake up well, then soak the parts for 15 minutes or so.
After soaking, usually any remaining carbon can be brushed off.
NEVER, EVER use any abrasive, polish, steel scraper (unless it's designed for that purpose), steel wool, sand paper or other abrasives or steel objects like pocket knives.
If the piston or cylinder gets scratched, carbon fouling will adhere and build up even faster.
Also, many gas systems use hard chrome plated parts, The finish can be damaged, exposing plain steel.
The reason not to put ANY lubricant, bore cleaner, or anything else in a gas system is that you have super heated plasma gas coming through at over 2500 degrees and some thousands of pounds PSI.
Anything in the system will get burned into a sticky tar-like mess that will cause the gas system to stick and cause jams.
I don't know of any commonly available lubricant or bore solvent that can take 2500 degrees and not burn.
Unless specifically stated otherwise don't use anything in a gas system, they're intended to run dry.
One of the very few exceptions is the M1 Rifle manuals call for using a few drops of lubricant in the gas cylinder where the piston rides.