The only sure-fire way to do it is to get the smallest *wound* piano or guitar string that you can find. Wrap it around two wood 1x4s and wear gloves so as to not slice your hands.
1: Wrap the end with the peg around the piece of wood. Get the end that normally goes in the winder peg - that is a thin bare wire - put that through the gasket. pull it through and get it wrapped around the other piece of wood.
2: The rest is a careful back and forth sawing motion with TWO people - one on each side. The tricky part, though, isn't the removal, but the cleaning the old junk off if you want to re-use it. To do that, you need to put it down somewhere. It'll crack almost any way that you set it down, so you'll also need a curved surface or some sort to put it down on that you've set up/prepared. An old couch with pillows and blankets can work in a pinch.
It's a two man job for sure.
