Lol wow, I'm shocked sometimes at the answers people get, to the point where I have a tough time believing them. But I'll take your word for it, I just can't imagine anyone would say "go on the website and figure it out".

In any case, here's what I would do...call them back, don't mention the previous agent or anything. Explain that you cancelled the flight, you were told the $100 would go on your CC (though, they really shouldn't cuz that could then just be used as a way to kinda milk your CC) but that they put it back on a gift card which you no longer have. I would say if you get the right agent they SHOULD (read SHOULD) be able to issue you an e-gift card for that $100 and somehow cancel the old one.

I can't say I've ever been in this situation, but I suppose it's a good idea to keep the GC until you've used the item you bought. The fact though that they do have E-GCs though may help you out big time here. If the phone call fails, maybe an email to AC customer service. Remember you're no longer looking to talk to reservations, strictly customer service, AC Res no longer has any reason to talk to you.