You cannot use the gift card on mobile order. Basically when you buy their gift card you don't get to enjoy their offers and deals because all those offers and deals on the mobile app.
Why can't I add my card on the McDonald's app?
If you are having issues adding your payment method, we recommend you log out of your 'My McD's' account, clear app cache, uninstall and reinstall the app, log back in and try again.