Gamestop may have told you to contact the manufacturer, but you need to tell Gamestop that they're the ones charged with giving a fully working copy of the game. If you puchased the game new and didn't receive a new fully-working copy of the game, then they need to refund or replace, as per their policy. Bungie will not replace any lost/missing/non-working/expired/etc codes. Bungie doesn't hand out codes, nor will they give you a code to replace it if you ask nicely, beg, plead, threaten, or complain. You need to deal with Gamestop, not Bungie, and if the person at Gamestop that you speak to says to talk to Bungie, then ask to speak to someone else at Gamestop. Only Gamestop can assist you, so whoever you speak to that tries to say deal with someone else, you need to ask to speak to someone higher at Gamestop. Do note that if you bought the game used, then you're out of luck and need to simply return it and buy it new.

Did you buy it used by chance?