Yes, gift cards work in the Oasis! I used one once and it worked perfectly fine!
What error is it telling you when you try to submit it?

Hmm, random double-post. Sorry about that!

Did you make sure that the card number is correct? You don't have to manually choose what brand your card is (Visa, MasterCard, American Express, etc.) as it does it for you whenever you enter in the card number. Have you also made sure you entered the 3-digit code on the back of your card, and/or made sure it was correct?

I would first contact Xylax before you did that, because I think that is against the rules, if I am not mistaken. Of course, I do see it as okay if you know that person and don't do it in the public of Lioden though! Is this gift card brand new, or did you find it somewhere like the kitchen junk drawer? Otherwise, I don't see the problem you're having with the card if nothing is incorrect and you are doing everything right. Are you in a restricted country?