Did you purchase it in a store where the card was displayed on a rack where anyone can tamper with them?
Some stores fail to protect your purchases by allowing scammers to grab a bunch of cards off the rack and copy the code down. The grey sticker can be easily replaced with ones purchased on Amazon, then they just put the cards back on the rack and wait for you to activate them. Ironically some of these same retailers will lock up products where they are exposed to loss if stolen, but don't seem to care if it is their customers that are losing money from a scam that occurred right in their store. In the future, only purchase Gift Cards from retailers where they are secured behind a counter.
You can try using the link below to contact Apple, but there is likely not much they will be able to do. I would contact the store and escalate it further to their corporate offices as that is what needs to change in order to protect their customers or stop selling the Gift Cards completely if they don't want to secure them.
If you can't redeem your Apple Gift Card, App Store Card, or App Store & iTunes Gift Card - Apple Support