On second thought, bringing a version without multiplayer to GOG would open another can of worms (and user rage). A shame, this one looked good (Age of Empires with Vikings).
As long as there is a meaningful offline single-player campaign, I personally would be fine with the GOG version lacking multiplayer, as I myself wouldn't probably touch the multiplayer part anyway, like I don't in 99.9999% of my games. That didn't stop me from buying e.g. Brutal Legend or Full Spectrum Warrior games from GOG, and various others with no working multiplayer.
But I guess some feel differently and there would be an uproar for the GOG version missing something from the Steam version. Oh well.
In fact, I belong to the school of thought that it is generally better single-player and online multiplayer games are separate games. You don't have to cram both game modes into one game, usually it is better they are not. Either concentrate on making the game single-player, or online multiplayer. Like Team Fortress 2, it doesn't even pretend to be a single-player game, even if you can play maps against computer AI bots, but that is more like a training mode for the real thing, against humans online. Also World of Warcraft, it didn't have a single-player campaign, did it?