Go onto the roof and see if you can pull the chimney pipe up after disconnecting it from the fire place, or at least lift it a few inches, then drill a half inch hole on each side and poke steel rod through it to hold it up while you remove the insert below.
The odds that this will work without further coaching are in the 40% range, we will learn something from your efforts however and can take more measures from there.
When I was doing that sort of work, I used an oxygen acetylene cutting torch to cut the metal away in pieces, that always works well, just be sure stand clear of falling metal or prop it up before you cut it, then pull the prop out with a rope when you are clear.
This will be step one, let me know how it goes. Put a plug in the end of the gas shut off valve under the house to be safe.
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