Eh, this is a weird one. The Postal franchise's entire raison d'etre is to cause controversy, to offend people. Be that the games, or the movie. Pee on people. Offend religions. Use a cat as a silencer. Randomly kill bystanders. All this because you can, not because there is any proper reward for it: be an offensive disgusting asshole just because this game allows you to do so. For what it is, it's fun, at least Postal 2 is for a while. I haven't played the DLC that has the GamerGate reference, but I'm not exactly shocked or offended by the fact the devs decided to throw that in too - whether they truly believe in it or not, after 9/11 jokes, rape jokes, etc. it feels almost mandatory for them to stay relevant. Mind you, I am not justifying it, and I have no idea about the devs' actual political and social views.
As for GOG's tweet, I also don't know what to think. It's unfortunate to extrapolate that from the game. I personally had no idea what that date represented, so I guess it could have been a honest mistake, but you never know intent. VG247's stance seems correct: if this was an intentional move, they're basically implying gaming journalism is trash, so why should they bother wasting their precious "trash" space talking about a company hating them? This with how CD Projekt treats their employees and views unionized devs is not exactly a good look. Seems fair to stop covering them at the very least until someone from GOG speaks up and gives out an explanation. Because Postal 2 is an offensive game, it should be, and they are right to sell it because it's a piece of gaming history for better or worse. But it shouldn't be advertised as "buy this game because it supports GamerGate and hates gaming journalism".