Folks, this is what we call a "fragile deal." It's not supposed to work. Someone used an obscure category when they inventoried some gift cards and the discount and coupons have worked. Target hasn't caught on yet because the volume of people taking advantage of the discount hasn't caused a problem.
If you call, you will ruin it for everybody. If you cause a scene in the store, you will ruin it for everybody. If you try to get an override by a manager, you will ruin it for everybody. If you see reports that the coupon works, but a cashier shuts you down, and you bring up forum posts as evidence that he should just try to see if it works, you will ruin it for everybody.
If you get shot down, move on. Try another time. Try another checkout line. Try another store. Do not try to force your way, or you will ruin it for everybody.
The coupon, and all other coupons, clearly state that they aren't valid on gift cards. The Disney cards clearly say, printed at least trice in the front, that they are "gift cards." Trying to argue that they are called something different by an internal Target system will only show that you're trying to take advantage of something that shouldn't be working to begin with.
If you abuse this loophole, they will notice. The 5% is a pretty nice benefit to begin with. Please don't kill it for everybody.