This is a very valid complaint, but I don't think putting the blame on GOG is deserved. GOG does sell many games that support online via Galaxy, some of them even offer crossplay with Steam and other gaming platforms. If this one lacks those features, I'm pretty sure it's the developer/publisher's decision, not GOG's.
Having said this, even though I do acknowledge lack of online features IS a valid complaint, I must say it doesn't really affect me, personally, as I wasn't intending to play it online, anyway. Plus, it is a very well-made game, even with the missing features, and I'd rather have the option to buy it on GOG with no online, than having GOG refusing to sell it unless the devs added those features.
Maybe I'd lower the price, since they're not selling the exact same product as they do on Steam, but, again, that's not really up to GOG.