Remove the bolt and soak it in a good solvent, then put a couple of drops of light oil behind the extractor and down beside the ejector.
I don't suggest disassembling the bolt unless you have the reassembly tool and know what you are doing.
Then remove the barrelled action from the receiver and take off the slide. With the barrel pointing down, use a penetrant (I like G96 Gun Treatment) around the gas piston. Again let things soak a bit, then reassemble. Don't oil the piston.