gift card from X

This seems to be some kind of promotion that X has given you and you will need to follow up with them on how you are suppose to redeem it. Apple does not have a promotion that gives you a Gift Card for subscribing to X. It would seem that they are giving you poor customer service if they tell you that you will get a Gift Card for subscribing and not telling you how you are supposed to redeem this promotion. To further direct you to Apple or even this Support Site for a resolution to their promotion was either a mistake on their part for not understanding their own promotion or worse is some kind of scam on their site that they don't want to talk about.

Do you have a link for the promotion that you saw? There is no Apple promotions where the details are provided on the Offers page, so you may need to follow up with X.