This is an unexpected but fantastic news!
Even for Power of Ten, there was nothing to suggest it had achievements, so another very good news.

I'd like to take this opportunity to report that the Galaxy achievements have also recently been added to the games Monster Prom 2: Monster Camp, Monster Prom 3: Monster Roadtrip, Baldo: The Guardian Owls and Rain World.

And it was discovered that DRAMAtical Murder and Togainu no Chi ~Lost Blood~ have achievements, but they are invisible and not officially reported on the gamecards.

Several Terminator: Resistance achievements are broken, specifically those that require multiple actions.

Easy Money - Successfully open 10 locks.
Hacker - Successfully hack 10 devices.
Historian - Find 10 notes.
Priority to Me! - Complete 5 side objectives.

Everything else, that requires a single action appears to be working fine. There is some talk that downgrading to version 1.028b fixes them, but this version is no longer available via Galaxy.

I was mistaken with the updates, I don' think Risen 2 and 3 have Galaxy achievements yet.

Added to the list, I really hope they fix them, there must be a version of the game where they worked because more than one person completed them.
Please try contacting the developers directly, possibly avoiding the publisher.
I'm afraid we'll have to wait for a next gen update as happened with the first Risen, hopefully it won't take long!

I logged a ticket with GOG and sent an email to Teyon. We'll see if this goes anywhere. Having googled this a bit, I learned that achievements can be quite glitchy on the PS4 version too. At the very least PlayStation can track the progress counter though. On GOG, there's no way to see how many were actually counted, or if it stopped at 1. GOG should really implement an achievement progress tracker.

Doom 3 (original) has an achievement percentage on the website. However, there's no other reason to think it's been built with support for Galaxy achievements - according to https://doomwiki.org/wiki/Achievement no version of the original Doom 3 has achievements - so my guess is that some settings were copied from D3BFG and this one was overlooked. Therefore, it will be stuck at 0% for ever.

Exactly, GOG accidentally added the "achievements" tag to the game, and the tag is what makes the percentage appear on the game. I'll try to report the incorrect tag, but so far they haven't fixed the ones I've already reported, so we'll see how long it takes.

I could not get any response from Teyon and, seeing how Terminator is never on sale here, despite having multiple sales on Steam, I suspect that it is more or less abandoned. Hopefully Robocop will fare better, if it comes here.

I did supply a screenshot showing my collectibles to GOG support though, and they have, thankfully, unlocked the broken achievements for me. I guess that's as far as this will go.

The Outer Worlds Spacer's Choice Edition has the same issue. Store and profile page shows achievements, but Galaxy itself says N/A. it does have achievements on Steam though, so hopefully Obsidian will fix this.
[EDIT01] Fixed now.

Would you say things are getting worse, or does it just seem that way to me? I feel like most of the time I see a new release, it doesn't have achievements and I immediately back away in disgust. Looking at the list of items currently tagged "New Arrivals" there are 101 in all. Out of these, only 34 have the Achievements tag.

Unfortunately in absolute terms the situation is very bad, you can see the annual trend of games without achievements on GOG (but which have them on Steam) in the "Charts" section of the spreadsheet. 2022 had a peak of as many as 184 games and 2023 looks like it will surpass it very soon.

On the other hand, GOG has also skyrocketed the number of releases, so perhaps as a percentage the number the games remains constant, but I cannot say for sure. What is certain is that GOG has signed contracts with some publishers of borderline shovelware games that release a lot of them, but never or almost never add achievements, like Meridian4 or Whale Rock Games, which impacts the statistics very negatively.

Thank you for pointing that out. I didn't realize my question was already answered.
It's the total volume that concerns me. It makes things worse on top of a stream of low quality releases. My wishlist is still overflowing and about 35% of the unplayed games in my collection don't have achievements. So it's not like I have no interest in anything released anymore, or that I flat out reject any game without achievements.
But I feel like I need to wade through a giant pile of refuse before finding something worthwhile. GOG was supposed to prevent that. And then when something interesting does get released, it's a game that's been on Steam for ages and they couldn't be bothered to bring it here with achievements. It's a very frustrating experience overall.

Rant over.

Apologies for the off-topic post, but if someone who got the Rite Denied achievement / Breach Duty mission in Deep Sky Derelicts could help me out, I'd really appreciate it. Dedicated game forums tend to not get much traffic, so I figured my odds of finding someone part of the 0.2% who unlocked that are higher here.

Since noone seems to be giving a damn anymore i will mention that achievs for Trails to Azure remain broken here as long as one decides to import a save from Zero.

EDIT: Mini rant: I have grown increasingly tired of stuff not working on gog. Be that Galaxy features or site features or whatever.
I still believe DRM-free is the way to go when it comes to pc gaming because seriously screw the alternative.
But gog makes it so so difficult sometimes.
/rantover

Aslaat has completed them all, so maybe he can give you some advice, I'll write him a message.
Sigh, I thought they had fixed this problem, but maybe they were only interested in Steam as usual.
I'll add a warning in the helpful hints section.

However, I assure you that we do care about these things here and will continue to fight to try and fix as many problems as possible.