I got the $200 giftcard with the purchase of an iMac, but got charged $200 on my credit card. What gives?

I can't answer your question, but I recently bought a new iMAC and traded in my old one. They gave me a trade in credit and a $200 electronic gift card which I used at the Apple on line store. It sounds as if they made a mistake on the new computer's price and then corrected that. (Hard to believe, though.) As noted, you need to call Apple. They should be able to tell you what happened.

You are not speaking to Apple here, please contact the vendor who sold you the iMac and ask your questions.

They took $200 off the price of the iMac and charged $200 to my credit card. This was not disclosed at the time of the transaction.