I would hire a contractor who installs gas logs, either the one who installed yours or another one. They should be able to make a house call and give you a free estimate.

My guess is that removing or capping the gas line and opening the chimney by re-installing a damper that can be controlled instead of one that's fixed in the closed position should be not so difficult. But there may be other things that were done to your firebox that could be more difficult to reverse. Better get an expert to look it over.

Then there's the question of what to do with the gas logs. I would junk them because I don't think that kind of system should ever be installed, and my conscience would not let me profit by selling it to anyone. But if you can use it in a patio on your back yard or other open space, that might be worth considering.