Hello Prosion,
Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are trying to add a GameStop gift card to the Wallet app of your Apple Watch. To help with this, please follow these steps from this Apple resource Add and use passes in Wallet on Apple Watch:
"Add a pass
To add a pass, do one of the following:
•Follow the instructions in the email sent by the issuer.
•Open the pass issuer’s app, if they have one.
•Tap Add in the notification.
•Tap a pass sent to you in Messages."
If you don't see instructions with the GameStop gift card, please contact them for further support of their gift card: Frequently Asked Questions | GameStop
Cheers!