If the back of your plastic gift card has a QR code, you can now add that card to your Apple® mobile wallet.
See below instructions based on iOS version of your apple device.

iOS10:
    1. Peel off the security label to expose the QR code.
    2. Open your digital wallet app and tap scan code.
    3. Scan the QR code with your Apple® device.

iOS11:
    1. Peel off the security label to expose the QR code.
    2. Open the camera app and scan the QR code.
    3. Safari browser will open with a tap to add pass message.
    4. Pass appears with an Add button.
    5. Tap the Add button and the Express pass will be added to your digital wallet.

Apple® is a registered trademark of Apple Inc.