The ad says in the fine print that the trade-in value of each game must be atleast $8. Which means you would have to trade in some higher priced games becuase Gamestops trade-in values are terrible.
Also have to make a note that its for Canada. You Might want to ask in the Canada Part of the forums
$8 * 6 = 48
Still a hell of a deal if its going on.
edit
Also check out the HAZE promo. That is crazy they will give you credit back if you don't like the game. Nice way for people to try the game out. Wish it was happening here.