You can use a gift card to buy anything in the itunes store try searching your question there!!
Can I Use A Itunes Gift Card Instead Of A Credit Card To Register My Ipod?
Yes you can first open itunes then go to the itunes store click on app store and scroll down the page you`ll see top paid apps and when you go down you`ll see top free apps and on the first app and click on it *P.S Your not going to actually buy the app . click get app and go trough the process and where it says credit card its going to say none or no credit card and thats all.