Thanks. Opened ticket 1331850. Supposedly long support wait times are over; I'll get a chance to see how true that is.

Edit: the good news is I got a response in under 12 hours, which is a first for me. The bad news is, it's now up to the product team to investigate further. In the past, that's the point where nothing gets done. Maybe things will be different now. At least the support ticket has been kept open awaiting an update, which hasn't been the case before.

Kane and Lynch: Dead Men is no longer incorrectly marked as having achievements.
Trash Sailors is no longer incorrectly marked as not having achievements.

A pleasant surprise to see support actually fix something for once. Here's hoping they'll make a habit of it.

Do you think it's worth telling them about https://www.gog.com/game/aragami_nightfall advertising achievements that aren't implemented? (I'd guess that the details were just copied from the base game, which does have achievements; except that if they'd just copied details over they wouldn't have the forum link pointing to General Chat.)

I'd also try contacting the developers about the missing DLC achievements. Though since the DLC is already listed in the Google doc, I suppose someone may have done that already.
But yes, I'd open a support ticket. I don't like incorrect information on store pages. The wrong forum link is also worth mentioning, since the game already has a dedicated forum.

I also tried to contact the developers some time ago with no results unfortunately.

GOG seems to report achievements in a game's DLC regardless of whether or not the DLC in question has dedicated achievements, so technically they might consider the feature to be correct.

That doesn't seem to be the case:
https://www.gog.com/en/game/hellpoint
https://www.gog.com/en/game/hellpoint_blue_sun

The point is that it's not mandatory for DLC to list achievements just because the base game has them.
Anyway, I opened support ticket 1335728 for Hellpoint - Sun, Aragami: Nightfall, and some miscellaneous things that bothered me about game store pages. We'll see if anything gets done.

Hellpoint - Blue Sun is now correctly marked as having achievements. No other changes (for now).
The support ticket has reached the "info has been passed on to the relevant team" stage, so I'm not expecting more updates. I will add that while the automated message said it could take 2 weeks to get a response, it was a pleasant surprise that it only took 4 days. Not exactly speedy, but certainly a massive improvement over the 5+ weeks from before.

Forgive Me Father is missing GOG achievements.

Thank you, I had them under observation because the publisher told me they were working on them, but so far, although they are visible, they aren't working yet. We'll see if they fix them with one of the next updates.

I have good news and bad news: the bad news is that the Whispering Willows achievements don't seem to be working; the good news is that I'm in contact with the publisher who is trying to fix them. The problem is that their QA team can unlock them without any problems, so more data would be needed to find the problem. If anyone has unlocked them or is in a position to test them (the first achievement is obtained in a couple of minutes of play) let me know.

Another small update, unfortunalely:

Dice Legacy
Achievements are broken, Devs promised to fix them in April but still did not do anything.

Gwent Rogue Mage
Completionist
Reported weeks ago, still no answer or fix.

Legend of Keepers
All Hail, Golthor
Expert recruiter
Mischief managed
Reported, no answer or fix.

Thank you, I added them all to the spreadsheet, hopefully they will be fixed sooner or later.

Lost in Play launched with no achievements (19 on Steam).

I'm playing Forgive Me Father and the achievements don't unlock.