Slide the gas cylinder out till it clears the splines. Turn the gas cylinder about 45 degrees and slide the piston out.
Don't feel bad. I asked the same question when they came in. Be very careful threading your gas plug back in. Mine cross threaded and stripped the first half of the threads on the plug.
Use the tool supplied to get it off.
I'm in the process of cleaning up my new 305 Shorty. The bayo lug seems to be in the way for removing the gas piston. Do I have to cut this down to get the piston out?