Hmmm, not sure why you thought they would be taking your credit card number if there wasn't a reason for it? I've eaten at CG numerous times and have had to listen to the spiel every single time regarding the noshow charge.
It should be common courtesy to cancel a reservation.
You've should have called to cancel your reservation when you decided you didn't want to show up. IMO anyone with holding an ADR should call to cancel regardless if there was a cancellation fee or not. I bet a few families got turned away that night because of you.
What I would do is call your credit card company and dispute the charge.
Not only is there a $10.00 cc guarantee at Akershus, but if you show up with less people than you have listed for your ADR, they will charge for the missing person. We were on the free dining plan this past Sept. I woke up ill one morning, but sent my mom and husband on to Akershus, just so we wouldn't be charged. Lo and behold, a $10.00 charge appeared on my credit card after we got back. I had not used that credit card down there and seldom use it here, so noticed it immediately. I called and was informed that the charge was because we had showed up one person short at Akershus. The fact that the others showed up didn't matter.
Your best bet may be asking if they have a recording of the call.