I have now a good experience in the achievements of GOG, and over time I have encountered numerous games that have them completely broken, or that have fewer errors in their graphics or title, so I decided to make a list of all the bugs and errors that I have encountered so far.

If you have more bugs to report, you are welcome, and I invite you all to contact GOG and as well the relative game customer support to ask them to fix the bugs as soon as possible. I have already reported many of them, but very few have been fixed and many other achievements are still broken, so I hope that more reports can speed things up.

This thread serves as an aid to players and developers to improve the experience of the games, so please don't derail in off-topic discussions.

Patias has created a Discord server where we can discuss Galaxy achievements, bugs, and help each other with the more difficult ones, with a small growing community, you're welcome!

https://discord.gg/g44CFpU

If you are even minimally interested in the achievements, please vote for these wishlists, they are of enormous importance to convince a dev or publisher that GOG users want the achievements for games that have them on Steam:

Show achievement progress
Achievement support for compatible games
Please add achievements for games that already have them on Steam
Improve the quality of Galaxy achievements icons

MISSING AND BROKEN GALAXY ACHIEVEMENTS
https://drive.google.com/open?id=1pDO6WTHLHyrrtidQ1MAxW6u8j3BxUaGcFaJsVyWj2QY

This is the master list of all the games that don't have the achievements on GOG Galaxy while they have them on Steam, and includes games with broken or partially missing achievements. Thanks to omega64 for creating the list framework.
As always let me know if you notice inaccuracies or errors.

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Glossary:

Invisible Achievements (Development Mode)
Achievements that work correctly but that the developers forgot to make visible through Galaxy or the GOG Profile. When a user unlocks one of these achievements, it becomes visible. This is a feature of GOG GALAXY SDK expressly created to allow developers to work on implementing the achievements, but it should be switched once the game is officially launched. It's easily fixed by notifying the developers or GOG support.
See the official documentation at the following link:
https://docs.gog.com/achievements/#setting-achievements-mode

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Helpful hints:

Absolute Drift
Achievements all work, but they are sometimes triggered with a delay for some reason. So always be sure to wait a few minutes before quit the game, especially if you think you should have unlocked an achievement.

Battle Planet - Judgement Day
Don't upgrade anything in the character selection screen, or the corresponding achievements won't be unlocked.

Control Ultimate Edition
"Crisis Managment" and "Ritual Initiation" are apparently doable only in a single session. The user Phoechan has written an exhaustive guide to get them easily, available at this link:
https://www.gog.com/forum/control/ritual_intuition_crisis_management_achievements

Deponia
Some achievements of the games of this saga are problematic or confusing. The user Ice_Mage has collected all the available guides and tricks in a handy thread:
https://www.gog.com/forum/deponia_series/bugged_and_annoying_achievements_master_of_switches_droggeljug_cheater_pinup_goal_ornithorhync

Evoland 1
Has achievements which the game keeps track of internally but provides no way for you to check what you have.
(Ask the developers to include the achievements for a complete gaming experience! support at shirogames dot com)

Evoland Legendary Edition
You must be online to unlock the achievements. Every other time you start the game, achievements don't unlock, so close and re-open the program until they start working. There are in-game indications for when an achievement unlocks, so if you don't get a Galaxy achievement at the same time, close and re-open the game without saving and try again. The Steam version has this same issue.

Factory Town
Most of the achievements unlock with a delay, and sometimes they don't unlock at all unless you follow a specific procedure. The user TheScorcher made a post about it here: https://www.gog.com/forum/factory_town/achievements_not_registering/post3

Flynn: Son of Crimson
'Hey There, Bax!' - find and speak to a certain character in 4 locations - this task needs to be done in a single play session, no quitting to the main menu or anything like that. The bug affects every platforms at the moment.

Journey to the Savage Planet
"We came, We Saw, We Kicked Its Ass" - The user A.M.R. wrote here a simple guide to unlock it easily:
https://www.gog.com/forum/general/list_of_broken_gog_achievements/post628

Mark of the Ninja: Remastered
Achievements can fail randomly: probably if you are offline or if the game crashes after completing a level. If it happens you'll have to delete the save file and try again.

Mary Skelter: Nightmares
Save the game before facing a boss, because you may not receive the relative achievement at the first attempt.

Metro: Last Light Redux
"Rabbit" and "Clean Escape" can be broken. Back up your appdata config file, then do a hard reset on both config files, then run as admin. https://steamcommunity.com/app/287390/discussions/1/350533172696204234

Of Orcs And Men
Achievements can fail randomly: Keep a save just before the moment when an achievement should be unlocked and try it again on a clean run with a very small number of saves, that seemed to increase the chances of unlocking.

Pizza Possum
The "Give me your chippies!" achievement of Pizza Possum seems buggy sometimes, even on Steam.
You can solve this by editing the stats.json located in \AppData\LocalLow\Cosy Computer\PizzaPossum\storage.
Just set the entry "NPC Furthest Distance From Home": to 0.0 and try to get the achievement again.

The Legend of Bum-Bo
It requires a mod to fix a bug which would otherwise make getting all the achievements impossible:
https://github.com/JeffAdriaanse/The-Legend-of-Bum-bo-Windfall

This War of Mine
Note: Malik on Duty may not unlock properly. The user Ice_Mage wrote a short guide to unlock this achievement and others:
https://www.gog.com/forum/this_war_of_mine/bless_the_radio_and_malik_on_duty_achievements_are_bugged

Trails to Azure
Achievements don't work as long as one decides to import a save from Trails from Zero.

Wolfenstein: The New Order
Achievements can fail randomly: if it happens disable cloud saves, delete local saves and config files and start a new game.

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Achievements broken known so far:

Expeditions: Viking (support at logicartists dot com) (info at logicartists dot com)
- Hero of Northumbria
- Mountain of Madness

Nova Drift (https://www.gog.com/forum/nova_drift/feature_upvote_our_new_home_for_feature_requests)
- Ludicrous Speed

One Finger Death Punch 2
Some achievements are broken:
- beat 150 levels
- beat the pixels bonus stage
- beat the cloud bonus stage

Predynastic Egypt
- Hospitable Host

Star Renegades Maybe fixed
- Short Seller
- Boardroom Raider
- Mobster Hunter
- Gangbuster

Super 3-D Noah's Ark Maybe fixed
- Arkitect: Part 1
- Over the Hump
- Arkade Mode: Part 1
- Pain in the Neck
And many other achievements bugged.

The Legend of Heroes: Trails in the Sky
Some achievements unlocks prematurely.

Train Valley
All unlocks prematurely:
- Collector
- Philatelist
- Juggler
- Mega-railroad

Continued in post 6

Not broken in the sense that you meant, but Evoland 1 has achievements which the game keeps track of but provides no way for you to check what you have.
The intention by the developers was clearly for people to look on their Steam account to see which achievements they have. Since it's not been integrated with Galaxy in the same way, there's no way for GOG users to see why the game says they only finished with 97.5% or whatever.

Celestian Tales: Old North has achievements that can't be attained at all.

Good to know, but shouldn't support\blues be contacted directly about this?

so you want a list of all the stuff that gog has screwed up on purpose so far?

Achievements partially missing:

8-bit Armies
Missing 1 achievement:
- Justice Seeker

Aragami (contact at linceworks dot com)
- Missing 12 achievements of Aragami: Nightfall DLC (see image attached).

Battlezone: Combat Commander (Discord: https://discordapp.com/invite/Rebellion)
Missing 18 achievements for unknown reasons (see image attached):
- Novice Scrap Collector
- Expert Scrap Collector
- Master Scrap Collector
- Novice Building Constructor
- Expert Building Constructor
- Master Building Constructor
- Novice Building Destroyer
- Expert Building Destroyer
- Master Building Destroyer
- Novice Craft Constructor
- Expert Craft Constructor
- Master Craft Constructor
- Novice Craft Destroyer
- Expert Craft Destroyer
- Master Craft Destroyer
- Novice Sniper
- Expert Sniper
- Master Sniper

Craft The World
Missing 6 achievements:
- Monster from the Depths
- The Sphinx’s Riddle
- Two heads are better than none!
- Defeat the animal within yourself
- No one will help the green-skins!
- Those born to dig can also fly

Distrust
Missing 1 achievement:
- Lucky ticket

FORCED SHOWDOWN
- Missing 17 achievements of Drone Invasion DLC (see image attached).

Heads Will Roll: Reforged
- Duelist
- Heads Will Roll
- Solo

House Flipper
- Missing 23 achievements of Luxury and Pets DLCs.

Minecraft: Story Mode
- Missing 18 achievements of Minecraft: Story Mode - Adventure Pass DLC.

Moon Hunters (info at kitfoxgames dot com) (Discord: https://discordapp.com/invite/kitfoxgames)
Missing 6 achievements (see image attached):
- The Final Victory
- Legendary
- Demigod
- Dancer
- Ancestral Artistry
- Chilled

Mutant Football League: Dynasty Edition
Missing 7 achievements:
- Blitzburgh Steelheads
- Cardinal Sins
- Motor City Maniacs
- Mile High Chronic
- Grim Bay Attackers
- Leaveland Burns
- Malice Hellboys

Offworld Trading Company
Missing 3+18 achievements:
- Algorithms
- Seat at the Table
- Off the Grid
-------------------------------------------------
- Drill x 100
- Surveying the Landscape
- The First Europans
- Branching Out
- Cave Dwellers
- The Search for Life
and many others

Party Hard (george.pinokl at gmail dot com)
Missing 1 achievement:
- Old Castle

Sword of the Stars: The Pit Osmium Edition
Missing 1 achievement:
- Game Night!

The Adventure Pals (Discord: https://discordapp.com/invite/ds53qjQ)
- Bake off (Roll back for unlock it)
Missing 8 achievements for unknown reasons (see image attached):
- Swordsman
- Cake Boss
- Tongue out of cheek
- Rejuvenation
- Headache
- Nine Lives
- Extinction
- Have my babies
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Achievements with missing icons:

Ashes of the Singularity: Escalation

CHANGE: A Homeless Survival Experience

Deus Ex Revision (contact at dx-revision dot com)
Many achievements are missing icons

Everhood

Gato Roboto

Guild of Ascension

Noch

Nocturnal

Old World

Order of Battle: World War II

Warhammer 40,000: Dawn of War II
- Hero of the Imperium
- Usurper

Warhammer 40,000: Space Wolf - Complete Edition

Wasteland 3

Ys IX: Monstrum Nox
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Achievements missing because the GOG version lacks some feature:

A Hat in Time
8 Steam Workshop, multiplayer and Steam Market related:
- Unlimited Possibilities
- Community Contributor
- The Community Thanks You
- Life of the Party
- Party Planner
- Party Animal
- Challenge Road
- Stick It To The Man

Airport CEO
2 Steam Workshop related:
- The explorer
- The contributor

Alien Breed: Impact
3 multiplayer related:
- Hard To Kill
- Team Veteran
- Time to Party

Alien Breed 2: Assault
4 multiplayer related:
- Co-op Campaigner
- Indestructible
- Rapid Deployment
- Time to Party

Alien Breed 3: Descent
3 multiplayer related:
- Meat Grinder
- Seasoned Squaddie
- Time to Party

Automachef
1 Steam Workshop related:
- Commit and Push

Banner Saga 3
1 leaderboard related:
- Eternal Arena Champion

Batman™: Arkham Origins
11 multiplayer related (dismissed also on Steam):
- Predator Paragon
- Killing Joke
- Fallen Knight
- Gotham All-Star
- Clutch
- Tales of Gotham
- I Like Those Odds
- Not An Ordinary Criminal
- Arsenal, Awesome
- Legend
- Know Thy Enemy

BioShock 2
14+1 multiplayer related:
- Unnatural Selection
- Welcome to Rapture
- Disgusting Frankenstein
- "Mr. Bubbles-- No!"
- Mother Goose
- Two-Bit Heroics
- Parasite
- Little Moth
- Skin Job
- Choose the Impossible
- Proving Grounds
- Man About Town
- Aqua Incognita
- Territorial
- Reincarnation

Blades of Time
1 multiplayer related:
- Out of My Way

Carmageddon: Max Damage
1 library related:
- Unholy Trinity

Deathtrap
1 Steam Workshop related:
- Steamroll

Deus Ex: Mankind Divided
17 multiplayer related:
- Breach: something

Doom 3: BFG Edition
6 multiplayer related:
- Telefragged!
- Crushed!
- Berserked!
- Ninja Killer
- Clean Sheet
- 2 Deaths - 1 Gun

Imagine Earth
1 Steam Workshop related:
- Workshopper

Mad Max
1 online functionality related (servers closed):
- Up to the Task: Complete all non-repeating challenges

Monster Prom
3 Steam Workshop related:
- Mod-tastic!
- Mod-alicious!
- Mod-tacular!

Mordheim: City of the Damned
9 multiplayer related:
- First Blood
- Rising Star
- Tactical Prowess
- Conqueror
- Testing the Waters
- Into the Fray
- Grizzled Veteran
- Battleborn
- /ragequit

Prison Architect
2 Steam Workshop related:
- Architect
- Warden

Project Highrise
2 Steam Workshop related:
- Mod Squad
- Experimential Experience

Redout: Solar Challenge Edition
1 multiplayer related:
- CHALLENGE THE WORLD

Saints Row IV: Game of the Century Edition
1 online functionality related (servers closed):
- Saintified

Sniper: Ghost Warrior
5 multiplayer related:
- Close Range
- Hold Your Breath
- Sharpshooter
- Supplier of Death
- Laurel Wreath

Sniper: Ghost Warrior 2
11 multiplayer related:
- Bloodstreak
- Pure Ownage
- The Sniper Elite
- Kill That F**ing Band!
- Blood And Tears
- Unhappy Camper
- U Can't Touch This
- Wrath Child
- Death Dealer
- Everything But The Kitchen Sink
- World Warrior

Superliminal
3 Steam Workshop related:
- Dream Sculptor
- Dream within a Dream
- Dr. Pierce's Protege

Supreme Commander 2
5 multiplayer related:
- Friends
- Good Friends
- Internet Commander
- Ranker
- Supreme Online Commander

This War of Mine (techsupport at 11bitstudios dot com)
2 War Child DLC related (for sale only on Steam for charity):
- Helping Children
- Street Art

Tomb Raider GOTY
15 multiplayer related:
- Adventurer
- Artilleryman
- Down Boy!
- Entrapment
- Escapist
- Good Samaritan
- I'm all that!
- Sole Survivor
- Lights Out
- Master Blaster
- Monkey Around
- Narcissistic
- On My Way Up
- Shopaholic
- True Commitment

Warhammer 40,000: Space Marine
25 multiplayer related:
- Armored in Glory
From - Battle Brother
to - Sector Cleared

Warp Frontier
1 due to Galaxy's lack of back end functionality to handle timed achievements:
- Family Time

Worms Revolution Gold Edition
8 multiplayer related:
- Worminator
- Social Butterfly
- Buddy, Buddy
- Scout Killer
- Soldier Killer
- Heavy Killer
- Scientist Killer
- Mars Rover

Worms Ultimate Mayhem - Deluxe Edition
10 multiplayer related:
- Loves Company
- Ranked Up
- Total Ranker
- Alexander The Worm
- I am The Worminator
- Finger of Death
- Join Me Luke
- Dedicated Ranker
- Feel The Power of The Darkside
- Cartographer

Worms World Party Remastered
1 multiplayer related:
- Bazookanade

XCOM: Enemy Unknown Complete Pack
2 multiplayer related:
- Meet New People. Then Kill Them.
- Bubonic

XCOM 2
1 multiplayer related:
- The Most Dangerous Game

Zombie Night Terror
2 Steam Workshop related:
- Starter Pack
- Creative Mind
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Achievements hidden because they were broken:

Adeptus Titanicus: Dominus
Bilkins' Folly
Breach & Clear (contact at fearthegun dot com) (support at breachandclear dot com)
Celestian Tales: Old North (This guy worked on the achiev on Steam: cipto at ekuatorgames dot com)
Decay of Logos
Gone Viral
Iron Harvest
Knightin'+ (The developer is waiting for an update of the Galaxy SDK to make the achievements operational)
Legends of Eisenwald (devs at aterdux dot com)
Maid of Sker
Moonlighter
Nocturnal: Enhanced Edition
Ocean's Heart
Rad Rodgers Radical Edition
Satellite Reign
Seed of the Dead: Sweet Home
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(emails for contact support have "at" for "@" and "dot" for ".")

A big thank you to all those who have contributed so far!

I agree, it's definitely a different sort of problem, I just thought you might want to include it as it's still broken. I think the game released here long before Galaxy and achievement support were present so it's not entirely their fault (although having an internal achievement system with no way to check the achievements is a bit of a facepalm worthy move).

Party Hard: "Lol, Really?" won't unlock despite finishing all other achievements.

That's a known issue with the game. Some of the other achievements unlock prematurely but you need to max them all out to get the last one.

https://steamcommunity.com/games/356570/announcements/detail/1607090108981221033

Of course we may not have that fix if the devs didn't send GoG the patch.

edit: Typo fixes on the last sentence.

Bugged achievements... as expected another thing that could go wrong will eventually go wrong.

Game: Turok: Dinosaur Hunter

The achievement "Turok: Treasure Hunter" cannot be unlocked
https://www.gog.com/forum/turok/treasure_hunter_achievement_a_known_issue

And it seems like the gog version misses 2 achievments from the steam version:
"Not Quite Mortal" and "Speedrunner"
https://steamcommunity.com/stats/405820/achievements

DOOM 3 has impossible achievements since GOG had to remove the multiplayer component.

If you want achievements in Deus Ex Revision, you'll have to roll back to the previous version. They won't unlock in the latest version. Gog support has been notified and they have acknowledged the problem, but no ETA of a fix yet.

These new achievements are only in the beta branch, it's not in the normal game yet, so i guess when the beta will become the new version, a patch will be avaiable for gog version.
When i have Galaxy overlay and nvidia geforce experience overlay active, none of them works with shadow warrior classic redux. Thats not a problem because the achievements are still unlocked, but no screenshots possible....i did not try it with deactvated nvidia overlay tough. I deactivated nvidia and the galaxy overlay ist still missing, so its a game bug, but achievements works perfectly!