Gift Cards can only be redeemed in the region that it was purchased. Was this a physical Gift Card you purchased and you also no longer have this Gift Card?
There is a standard Support Article related to most issues with Gift Cards, but you might need to use the link in there to Contact Apple directly as it may depend on many factors, if possible at all:
- Gifted to you from App Store
- Purchased online at apple.com
- Purchased in a store
- Do you have the purchase receipt
If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support