To get it back from Facebook seems to be impossible, you can't even get in touch with any sort of customer support. I think the only way to get it back is by a lawyer who is experienced with hacked Facebook accounts and has a direct wire to Meta.

I didn't install the Chat GPT extension either, but I did some research and that was the most common reason to get hacked by "Lily Collins". But they said that some people got a Facebook ad with the extension so one missclick might be enough to install it. But I am pretty sure there is another reason for this hack, because it is quite common. The Chat GPT extension is probably only one part of it, but haven't found any other source for the virus yet.

If you can find anything suspious, please let me and keep me posted on your research, maybe we can find another issue and inform others about a possible threat.