I believe you need to register the Visa gift card online first and then you can make online purchases. Right now it's a card but not attached to a name. You can use them in stores without registering the card but not online. At least, that's the way it is where I work.
Also, check the fine print on the gift card. Ours is free for the first 6 months but then after that, they charge you a $1.50 fee. Bogus I agree but I always tell people before they buy it so they know.
HTH