All the content is included in the GOG build. I think the only reason they present them as "Free DLC" on the steam store is because the Wargame community is quite split between people who just want to play single player and people who are hardcore about multiplayer.

The multiplayer crowd has no interest in yet another singleplayer campaign, and singleplayer buffs do not care about new multiplayer content. Instead of forcing both camps to download stuff they don't care about through game patches they made it optional through the DLC interface on Steam.

Well there could be an even more cynical explanation of course, maybe they just want to make it look like you get more value with "free DLC" on the Steam store, a way to indicate to attention-deficit steam users that the game is not dead and there is a reason to buy it and play it.

Either way, what is important is that this stuff is already included in the GOG build because it might as well have been a part of the original game to begin with.