What should I do if my Apple Gift Card is stolen?

See the following from Gifting - Shopping Help - Apple

Lost or Stolen Cards

To report a lost or stolen Apple Gift Card (or any previous-generation Apple gift card), please contact Apple at any Apple Store location or by telephone at 1‑800‑MY‑APPLE. Replacement cards will be issued after Apple verifies that the lost or stolen card has not been redeemed and you present your original purchase receipt. Apple is not responsible for lost or stolen gift cards.

If you are not in the USA I would advise you to go to the article in the link to get the support contact information for your country. I am pretty sure that 800 number is for the USA only.