I already own Neverwinter Nights 2 and Planescape: Torment, both bought here on GOG. If I purchase the complete D&D game pack - can I gift the extra copies of the games to my friend?

Yes, the best way is to send it to your e-mail.

If you simply click buy now on the deal page it will only add what you don't have and give you the price based on that. You can also buy a gift of the whole set if you do want to gift the others to your friend, but then you would redeem the gift (what you need) and then the others would be left on the code which you could pass on to your friend.

Yes.

You have to gift the entire set to yourself first. Then redeem the key that you receive. You will get the games that you did not have, and the games that you already had will stay on the key. Then give that key to your friend.

Or, just buy the remaining games for you, then buy those two games for your friend... that's two transactions.

Excellent - thanks for the quick answers guys, appreciated.