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Darksiders 2 features a online feature, where players can mail items to each other. You can even check online Highscores. Is the gog Version only offline?
Yes, the GOG version from Nordic games is offline. however when you buy the DLC pack, not only do you get the extra stories and the crucible. But when you check your mail totem ingame you'll get all the armor and weapons that were promos.

Hope this helps. The game is fantastic with or without the online so its no big deal :)
Yes, you cannot use the "Online Mailbox" game feature/functionality. Which means, no item trading between players. Not a terribly big issue of a deal, though. But this was the only way to get the Crow Armor Set, which now is inaccessible, at least in GOG.

The real deal is the inability of the game to recognize you beat and played the prequel. If we had achievements and 2 could "read" those from 1, like in Steam, two unique items would be mailed in 2: "Pauldron of the Horseman" and "Chaos Scythe Fangs". I really wanted those two (cosmetics, mostly), i bought 1, i played and beat it and i cannot have the bonuses for having done so...