I am glad of this partnership. Paradox is the most player friendly and honest publisher i known.
Steam haters' points about Paradox's stand at DRM is wrong. You can copy Crusader Kings 2 and Europa Universalis 4 files to a steam-free PC and play single player and with their older games you can even play multy player through this method.
I meant Steam edition. Since regular GOG bonuses like OSTs, books etc. are reward for pledging, i do not expect any bonus for GOG edition for at least at release.