I found some odd info on the Sekai Project forums, after asking if they have any plans to release Grisaia on GOG as well.
And people mentioned that even if they want to releasing them on GOG, the problem is that GOG are refusing to accept them.

So, why are Visual Novels being refused on GOG. Does anyone know?

Nobody knows. GOG is tight-lipped on that matter.

As much as I like their curated approach, I'd like them to be more open about their acceptance criteria.

Currently Im reading/playing "The Hero of Kendrickstone" Demo with a view to buying other games in the series. Im impressed with what I have read so far and will probably buy it and Tin Star tomorrow.

I don't know if they are "visual" novels as they are only text, but,Im surprised GOG don't have them when you consider they have movies.

It it curious, isn't it? I mean, Hatoful Boyfriend and Long Live the Queen are here, there may be another as well, but yes, I think there are several visual novels that should be here that aren't.

One reason may be Japanese publishers not working very well with world-wide distribution. Many of them seem to completely abhor the idea that Japanese gamers may want access to PC versions of their games and would prefer that only console versions were available, so they frequently restrict PC distribution to only outside-of-Japan while GOG tries to have world-wide distribution (barring legal issues such as a game being banned, which isn't likely the case here).

why shouldn't they refuse. there are plenty other stuff to do, before even considering to sell vn's here

Well, we can piece the puzzle together. We know that they tell people "yes" or "no". We know that some of the reasons for 'no' are "I liked it, but no" and "It's not our style" and "We don't think it will sell well enough here". We know that some of the reasons for 'yes' are "I liked it, so yes." and "Lets add this game to our catalog." and "We think it will sell well here".

We also know that votes have no impact on this acceptance criteria. It's not about money, it's not about image, it's not about fun, it's not about popularity - there is literally nothing that other platforms would use that you can count on GOG to incorporate consistently. What works for one title would not work for another.

So in conclusion we can assume that they defer it all to the team they hired whom seem to decide everything on a whim rather than on any logical methodology. They were probably hipster interns that turned into power mongers after a while. If they got fired GOG would make much more sensible decisions for sure.

Not that I even like Visual Novels personally. I think they're all crap. But they're popular crap that sells well and they don't require as much maintenance as other games genres. This should mean less work for the compatibility team...it's a no brainer.

Well, Long Live the Queen is more complicated than usual visual novel with its character building so I would understand that even if there were no other VN here. But if HAtoful Boyfriend is here then I don't see why other shouldn't be present.
Maybe it is just that the ones that want to be here are not up to GOG standard and the ones GOG would like are not fine with being here?
But maybe GOG is more strict in case of visual novels than other genres.

The most likely reason why Hatoful Boyfriend is here, is because it's published by Devolver Digital. Otherwise, I doubt GOG would care that much about it.

Believe it or not, Visual Novels have a pretty decent sized fanbase, and if GOG chooses to alienate them, it's a real shame. But it's GOG's call, maybe they don't want to get more customers and money.

Does GOG even have a style these days?

I'm sad for GOG.

I'm honestly unsure about it myself, being a developer.
As others pointed, if a game like Hatoful Boyfriend is here, there is really no reason why other VN/dating sims can't be here.
Even more strangely they rejected both my VN/RPG (Loren Amazon Princess and Seasons Of The Wolf).

Now you can check them for yourself:
http://www.winterwolves.com/lorenamazonprincess.htm
http://www.winterwolves.com/seasonsofthewolf.htm

skill trees, stats, inventory, randomized items, quests, 100+ different enemy types, etc. Yet, they were rejected too.
So it's not just about normal VN, which I could "sort-of-understand", but even VN/RPG which are quite popular now. I thought might be about price, but nowadays with the frequent sales, it doesn't really make much sense to reject a game because of high price.

Mystery! :) maybe one day they'll do like Steam and open up more.

How is "visual novel" defined, in that context ?

Are visual novels interactive (besides clicking the 'next' button) ? Are "fighting fantasy" books visual novels ?

Never saw the appeal in these types of games anyway.