I will 100% confirm that in Canada you can use gift cards/itunes cards to buy apps. Even if the card says you cant...you can. This change was a while ago.
http://www.iphoneincanada.ca/iphone-news/canadian-itunes-gift-cards-now-work-for-apps/
Saberon makes a good point that I left out. The gift cards you purchase in Canada will likely say they are for Music and Videos only however, as mentioned they do work for apps from the App Store.