Cork the barrel at the chamber, top the barrel off with Kroil, WD40 or LiquidWrench from the muzzle end and let it soak for a day. Then tap lightly on the outside of the gas cylinder with a plastic mallet or a piece of hardwood. Get a 1/8" - 1/4" thick strip of aluminum or brass, file a sharp edge and rest it against the rim of the gas cylinder right next to the gas block. Tap lightly on the metal strip with a hammer in an effort to drive the cylinder off the piston. Use several locations around the perimeter. Once the cylinder starts moving, carefully tap it back towards the gas block and start over again. Eventually it will move more and more with every tap, until you have it completely off the piston or far enough so you can see the hex on the gas piston and remove it with a wrench for further disassembly. Clean the gas piston and remove rust from inside the gas cylinder with steel wool or fine sand paper. I went through the same motions with my AC45 and it came off after about an hour of careful tapping. None of the parts has been marred or otherwise damaged.
There was a discussion a while ago on this or another forum regarding the logic of coating gas cylinder and gas piston with oil. I think the situation you're dealing with right now has not been caused by a liberally lubricated piston and cylinder. Quite the opposite is true, I assume.
