From FlyFF WIki
Gpotato is the currency in the Cash Shop. You use gPotatoes to buy Cash shop sets, and items. In order to purchase gPotatoes, please log in to http://www.flyff.gpotato.com with your account ID and password. Once logged in, click Store in the little toolbar. On the left, look for the little button that says "Payment Option". From there, you choose which method you want to use to purchase gPotatoes.