I don't see where user can enter their gift card code.. any idea on how to enable it? - Thanks. 

Gift cards can be redeemed at /store/redeem where the value is added to the redeemers account credit.

Thanks for that URL. Is their anyway, it can be added during payment option? All the commercial websites, accept Gift Card during payment processing... this is very odd way, to ask customer, first redeem your gift card and then buy things with it. 

This is actually quite a normal process, as this is a gift card, not a discount code. For example, if you get a gift card from amazon, you have to redeem it in the same manner