I read somewhere that GOG throws a lot of Japanese games out even though publishers and developers are willing to put them up here.

Apparently stuff like Monster Monpiece and 100% Orange Juice both got turned down by GOG. Is there any reason behind this or is it just GOG hating on Japanese games?

Yes I understand that there's Trails, Ys, and Neptunia. But That's only two companies and Neptunia is limited to only the first game out of 3 that are on on Steam

region locks it seems

Ask the developers, they get a reason for rejection. (even if it's a bad reason)
GOG will stay silent on the matter.

We already have region locks from the point of sale on some games here (https://www.gog.com/mix/regional_locked_games_on_gog). If the only thing keeping certain games from release is Japanese publishers not wanting their games available to their own countrymen via GOG (for whatever convoluted reasons), then bring on the locks. It might be a bummer to Japanese customers but they'd be no worse off than before the releases and there are ways around region blocks. ;)

GOG considers many Japanese games too unknown outside of their home country to be viable to sell.

Yes this reason is bullshit.

Also, for the ones unwilling, Japan loves control on their games. Recall if you will Nintendo and how Playstation had to get a western lead.

how did the playstation get a western lead ?
the nes was sold as a trojan horse by pretending to be a toy

Well, having Mark Cerny as the console's lead architect was more what I was getting at, but you have to get the idea that behind the scenes, Playstation's policy towards the publishers and developers was much more friendly than Nintendo's. You'll have to recall all the masks and disguises all the companies had to make under the NES era, such as ULTRA Games (Konami).

oh your talking about the ps4

i was thinking in terms of the nes snes and ps1

the ps1 didnt have much of an uphill struggle i think

The PS1 didn't have an uphill struggle because Sony saw and experienced what Nintendo did. They voluntarily chose not to be hard on publishers. :V Also, they chose a format that was easy to make, cheap to boot, and easy to program for. Unlike cartridges.

Because GOG doesn't appear to know anything about PC gaming outside of Eurojank (tm), and doesn't particularly care to change that.

Too niche outside of Japan, apparently.

GOG turns down loads of games they don't think are up to their standards. 100% Orange Juice is a digital board game whose main draw seems to be reuniting characters from other games in the developer's catalogue, games that are not on GOG in the first place. Monster Monpiece is a card game about monster girls in skimpy outfits known mostly for making players jerk off their PS Vita to open card packs.

GOG doesn't hate japanese games. They have stuff like Trails in the Sky and La Mulana, but to come to GOG those japanese games need to be good in the first place.

Actually... You jerk off the vita to evolve the girls into stronger versions with skimpier clothing. :P
I wouldn't call Monster Monpiece a good game though even ignoring the fanservice.

I wouldn't go as far as suggesting that GOG's only turning down bad games. Their blocking very highly-regarded games in certain genres (primarily shmups) is well-documented at this point.