That goes the other way. Can buy old games such as Startopia on GOG in Japan, but not on Steam. But yep, seeing as even the Square games we can buy on Steam in Japan are not available on the likes of GMG and GOG are becoming more like GMG -- in terms of both "promises" and "actions", such retroactive restrictions here wouldn't surprise me either.

Shouldn't even be that. In the past different publishers meant GOG didn't sell worldwide until they had a deal with ALL publishers -- they even used to temporarily pull titles from the store worldwide in the event of a regional issue. But I see they went the easy route and silently dropped that in favour of only restriction in main markets when legally required, but Japan is open to the whims of the publishers once again. That or GOG simply lack the ability or desire to even try to negotiate Japan releases. But even then, Dying Light is purely a GOG fuck up. They didn't know how to handle a different release date for a single region and now can't own up to their error.

So they leave the publisher unable to make sales on the main game in Japan while still selling DLC that is completely useless and thus not going to make any sales in the region. Yet, GOG refuse to even remove the pointless DLC from sale. This joke situation is something we'd expect of Steam and even then, only with Japanese publishers.

As far as any legal basis for a game not being available in Japan (and not purely because of a different publisher), there is none for PC. There is no compulsory requirement to rate PC games through CERO. As such even games that could ultimately require alterations for sale on console can and are sold unaltered on PC in most cases. So there is only one reason for a game not being available on the format in Japan when it is available elsewhere: the publisher.

For the Neptunia games this is a bit complicated. Idea Factory / Compile Heart (essentially the same company) handle releases on console. Idea Factory International are responsible for the development of the PC versions. As such Idea Factory International hold the rights for those versions. Seeing the success of MegaDimension Neptunia VII on Steam worldwide, Idea Factory (Japan) finally started listening to requests to sell PC versions in Japan. And they ultimately did that for Re;Birth 1 through 3 (but not MDNVII) on Steam. That is why the Steam versions can be enjoyed with either Japanese audio + Japanese text, or Japanese / English audio + English text. So it's not entirely unreasonable to expect GOG to also be able to contact Idea Factory (Japan) and negotiate the release of those builds in Japan.