yea you can order the dnd set and it will give you a recalculated rate depending on what you own already for example I can finish the set for $17 instead of the set price since I own a couple games already.
You should have a little white banner over the bundle that will tell you your new price to finish the collection.

Gets you every time!

With GOG, any game you currently own on GOG counts toward the full deal. So you don't need to repurchase NWN2 to get the full bundle (as you would on Steam or humble bundle)

hmmm basically i was looking to see if it was possible to get the rest of the set for whatever the difference was but im guessing thats not gonna happen lol

That is exactly what will happen automatically. :) Simply go here: http://www.gog.com/promo/summer_hasbro_bundle_130614 any you own will already be marked and accounted for, and you will not have to purchase them again.

eh was looking to only pay the like 3 dollar difference but from looking around that probably wont happen :(

teaches me to buy games at full price lol

You know, it's still probably worth contacting support over it. GOG is really keen on customer satisfaction, and you just might have a case...

If you haven't downloaded the game or any of its contents, and it hasn't been more than 2 weeks, you can get a refund.