Can anyone tell me how to remove an old portafilter gasket?

Start with a pick tool or small flat blade screwdriver and try to just pry it out that way. If it is really fossilized, you can use a Phillips to screw a small wood screw into the gasket at a 45 degree angle. That will usually pop it up enough to easily get your pick tool or screwdriver under it and you can often get it out in one piece that way.