Read terms and condition but I would be surprised if Amazon Gift card was not accepted for shipping to Poland. I think the gift card can be in any currency, Amazon should handle currency exchange. And the best would be pounds if they want to order from UK or EUR if from Germany. But they shouldn't loose much on currency exchange if it's USD.
And as json wrote, we don't use Amazon giftcard because Amazon is not very popular in Poland, because they don't have polish version of website. But ordering from International version (.com) or German or British is possible to Poland and gift cards should be accepted then.
so the best buy them this one, because it seems that only British amazon ships shooings above 25 GBP for free to Poland.
Eligible European Destinations
(Belgium, Denmark, Finland, Gibraltar, Iceland, Republic of Ireland, Luxembourg, Netherlands, Poland, Portugal and Sweden)
Spend to qualify: £25
Can I get FREE Super Saver Delivery if I redeem an Amazon.co.uk gift certificate?
Yes. As long as your order qualifies, you can use your gift certificate and receive FREE Super Saver Delivery.