Sheershaw: 1) It's difficult for developers to maintain update and feature parity on GOG with Steam. This is something you hear repeated from devs, that maintaining the GOG version is extra work, which isn't worth it when the sales of games are pretty marginal.
I wouldn't be surprised if it's this factor, rather than the DRM free part, that keeps major games off of GOG. GOG should focus on making sure that publishing a game on GOG is as cheap and painless as possible, so that devs don't feel much regret over getting a game here even if the relative sales compared to Steam are low.
2) Their "curation" is incoherent at this point. Hentai RPG maker games get approved, but actually interesting looking indie games that lack high production values get rejected.
GOG should loosen their curation standard a bit--reject the obvious meme trash, maybe with a $10 price floor to discourage 99 cent asset flips, but otherwise being more liberal about what sort of games they allow.
They can still maintain their current standard of "curation" in a more indirect approach, like by actively promoting games they actively want to sell, but streamline the process for actually releasing a game on GOG.
3) GOG Galaxy was a mistake, but it's one they will have to live with at this point. I think their idea was that with stuff like the Epic store and Battle.net, gamers would want a way to consolidate all of their launchers. If GOG provided that, gamers would inevitably click on the "Store" page on the left hand side of the client and in turn buy games from GOG.
I actually don't think it sounded like that bad of an idea, but in retrospect, it didn't pan out that way. The gamers who don't care about buying games through stores other than Steam generally don't care about having a billion launchers, and generally launch their games through desktop or start menu shortcuts anyhow. The one who do care are either Steam supremacists, or are using a superior client like Playnite.
Consider checking out this Reddit thread about updating on GOG. I
think the biggest issue is just that most people just don't have a reason to use GOG (other than "old games on new machines", they don't care about DRM-free; but now old games usually get released on Steam too, often GOG versions of them), thus GOG can't improve their market share and userbase, thus quite a few devs and pubs of all sizes (but perhaps the biggest companies especially) are unenthusiastic to bother releasing here or low sales make them "lazy" to update here. Perhaps in Japan the DRM thing is still very important, though SEGA had games DRM-free on Dotemu store like 12 years ago (when Dotemu
had a store).
I don't think Epic Game Store was even a thing when Galaxy first released ~7 years ago, or at least EGS wasn't being aggressively pushed and actively trying to take on Steam. The sense I get back in 2015 is that with Witcher 3 being so beloved and popular, and W3 box codes only redeemable at GOG and bringing hundreds of thousands of new people in, there was a feeling that GOG could actually build into proper competition to Steam. Obviously that didn't happen for various reasons, including Fortnite blowing up and EGS pushing their way onto the scene, but 20/20 hindsight...
I don't think Galaxy was inherently a mistake - I
believe it was said some devs like Telltale wouldn't release on GOG unless there was even an optional client for feature parity (achievements), and
iirc on a survey GOG asked people if they wanted a client and something like 70% of people said yes,
but I might be misremembering the survey thing. Maybe Galaxy 2 was a mistake and trying to be an "all in one" client.
I've already said it many times in the past few months or years but I'd like less curation and more games here in general, I don't know GOG's decisions about that unless they're focused on releasing games that don't need updates or are from "trusted" devs or publishers ... ?