That's weird, it might be worth asking them if it's not a mistake, especially considering that the game is rather "old" and IF doesn't seem the most "draconian" Japanese publisher when it comes to DRM or region restriction.
Still waiting for the remaining games though...
It's a bit more complicated than that. The publisher for the GOG release is a third party (Retroism, IIRC) not involved in the Steam releases. At the time of the GOG release, GOG was the only first party site people in Japan could buy Re;Birth 1 from. At that time, it was regionally restricted on Steam. Although we could still buy it from Bundle Stars and Humble.
Also at that time the publisher for the Steam release was Idea Factory International and they *did* refuse to release it in Japan stating they didn't have the rights to do so. When Idea Factory Japan were asked about it, they stated they had zero connection to the PC release and that it was 100% handled by Idea Factory International. That in part might explain why the third party publisher stepped in for the GOG release.
However, following the release of Megadimension Neptunia VII on Steam, Idea Factory Japan realised that Steam was a viable outlet for their games in Japan too (basing this on the rest of world sales of Megadimension Neptunia VII). In keeping with that, they finally released Re;Birth 1 on Steam in Japan on August 25th. At that time the Steam version for all regions was changed to allow for full Japanese language support.
I take it the GOG version remains English language only (except for Japanese audio).
So now there are two publishers for the Steam release: Idea Factory International and Idea Factory Japan. This is also why Re;Birth 1 Steam store page now shows both the English and Japanese titles. Incidentally, following its Japanese Steam release, Re;Birth 1 was the number 1 top seller on Steam in Japan for the entire duration of its launch discount. Even outselling Earth Defense Force 4.1, which has been hovering around second place for over a month now.
Re;Birth 2 and 3 will also likely get full Japanese support too on Steam in September and October respectively as Idea Factory Japan are timing these releases to match the PS Vita re-releases they are currently doing. With that in mind, I honestly don't see GOG getting any future entries.
Now in the case of Dying Light, this is all on GOG. They fucked up. And now it seems they are too bloody stubborn to speak to the publisher about their mistake. So we get buggered over as a result. Then again, it'd be cheaper to buy on Steam anyway...
No, on Steam it's published by Idea Factory International, on GOG by Retroism.
I would bet this restriction doesn't come from GOG, but from the publisher. [...].
Perhaps Retroism doesn't have the right to sell to Japan then.
Do note that in the past when GOG have had a rights issue with a region, they've pulled the game from sale worldwide. If this is a rights issue, it's the first time they've gone against their one-world principle. But given how they threw their one-price principle under the bus, it wouldn't surprise me. GOG are fast putting publishers first, customers last.