In my opinion, all the goods that you buy on giffgaff are taxed, but I may be wrong because I am only passing through Europe. My permanent residence in Austin. In general, I would advise you to write to giffgaff support or contact a specialist. However, if you don't have the money to get advice from a specialist about this, then read the articles on this site -- https://www.greedyrates.ca/. There are various articles here about how to manage money. And I think I saw an article about taxes there, maybe that's what you need. I hope I have helped you. Good luck to you, friend.