I'm not saying your wrong about being able to use it. But the concerns aren't taken away by this advice. It's even worse, because that cashier may just decide, right after you bought the gift card, and before using on your items, that they won't accept it as payment. Not likely, but certainly not out of the realm of possibilities.
I also think the better strategy is to buy the gift card, wait till you get the bonus points, then use it a little at a time. Buying and immediately using on other items quickly may trigger some PCO employee looking at that, and either disqualify the bonus points, or even worse clawing the points back. (This has been posted as happening if PCO decides it's an abuse of a promo.)
And for those with the PC World Elite C.C. who get 3% extra in points anyway on SDM purchases. Would that in effect be forfeited, by buying the gift card first, thus making this only a net 7% bonus?