If you are a new customer then you will be able to receive your bonus gift. But you will receive it after you made all the transaction/set up account. So you can't use it to pay down $100 upfront cost.
Note: $100 upfront means you have to pay $21/mo tab charge for 24 month and you have to pick a plan from one of the Tab plus plan.
Retailers usually won't use their gift cards to pay down the phone because it somehow comes out of their profits. I don't know how, but that's what upper management at Mobile Shop told me when I was in Uxbridge. Some may give you the leeway during Christmas time but generally the policy is no.