"Sometimes, and too often, publishers even refuse to offer versions of games that have Linux native options available on GOG."
There. I fixed that for you.

I'm happy with GOG as it is with regards to Linux. I can download my installers hassle-free regardless if they're Windows or Linux, and the Windows installers work nice on Wine. What more could I ask for?

I've been playing my GOG games on Linux just fine. Heroic Launcher is great. You can use it to install your games and play them offline, and it will also take care of GOG achievements, cloud save sync, and tracking time played. And now with UMU launcher allowing you to use Proton outside of Steam and eventually the community will fill out the UMU / protonfixes database with more GOG game specific fixes, GOG doesn't really have to do anything.

They're not as big as Steam to where they can make such an investment into trying to support and increase popularity of an OS. Maybe they can occasionally contribute to Heroic Launcher and advertise that, but I don't know what else they're expected to do for "Linux support."

Edit: From what I've noticed, a lot of the official Linux versions of games, are usually abandoned or don't perform as well as the Windows version running through Wine / Proton. So I'm not sure how useful it would be to have those versions provided. Of course I wouldn't mind if they were, nothing wrong with more options. But if they are abandoned, maybe the devs don't want to offer support for them so when they bring their game over at a later date to another store they skip the abandoned Linux version.

They could do more towards that not being the case. There are plenty of titles for which GOG users are treated as 2nd class citizens one way or another, shouldn't also have a lower class of GOG users, making them third class...
When you get a game with multiple supported OSs, you should be able to rest assured that you'll get the same thing on any of those OSs. You're paying the same, after all...

Sure, but that's not a GOG only issue I think right? I was just talking in general. Doesn't this issue exist on Steam as well, with Linux ports being abandoned at some point and not being updated like the Windows version? Or just outright dropped. I seem to remember that being the case for Dying Light, but maybe I'm wrong.

It is an issue. Some publishers on Steam have dropped Linux support with some even stating to just run the Windows version under Proton.

Many games have a native Linux build on Steam but are sold only for Windows on GOG. Even games released the same day on both stores.

So there clearly is a "GOG only issue" here, or more specifically a "DRM-free only problem".

... which is pretty absurd to affect the more open and free OS...

Huh, I wasn't aware of that. That is weird. I wonder who's making that decision then, GOG or the developers for whatever reason.

Depends, Feral Interactive is an example of a very pro DRM developer so none of there Linux ports have appeared on GOG.

Of all the cases I know about, the culprit is always the publisher (the developer for self-published games). I can’t think of even a single exception.