You can't and don't. It's virtual accounting. They're saying the game is $40, and with GCU 20% discount it becomes $32. You get a $10 gift card towards a future purchase, so it's "essentially" as if the game only cost you $32-$10 = $22 even though you're still paying $32 out of pocket.
That is what I thought. The way jacknicklson explained it, I interpreted it as if you could use the e gift towards the purchase of the game and still receive an extra e gift card. "By including the e gift card as a discount on the purchase price. Then when you use it, you get another $10 off, its practically free money!" Thanks.