If you purchased the gift card on a rack at the retailer, you were scammed right in that store and it is the retailer that failed to protect your purchase. The scammers grab a bunch of cards off the rack and write down the codes, they can replace the sticker with ones purchased on Amazon. After they put the cards back, they just have to wait for you to activate them through the check stand and then spend the balance before you get a chance to. The retailers know about the scams that happen in their stores, but some fail to protect their customers.
You did the right thing by contacting the police and I also recommend filing a report with the FTC and file a complaint with the retailer that sold you the card:
https://reportfraud.ftc.gov
In the future, only buy Gift Cards when they are secured behind a counter, or purchase them online at Apple.com.