How can I buy gems using iTunes gift card?

How can I buy gems using iTunes card

Redeem the card onto your account, and click on 'buy' in the app - your account's balance should automatically be charged for the purchase.

If you create a new account, as per the HT2534 page that you posted from, then you will need to delete the and redownload it with that new account - all content is tied to the account that bought/downloaded it, so only that account can do in-app purchases in the app.