This list serves the completionists to find out which games are missing from the series that are already on GOG. You may vote for the missing games to demonstrate your wish for a release. Please let me know if I miss anything.

This game is, to keep it down in size and make it more realistic, for PC (Windows/DOS) games only. If you see something on the list that shouldn't be there, let me know!

Missing expansions, spin-offs, official or unofficial remakes are all welcome to this list. However inferior "original" versions (Grim Fandango, Monkey Island, etc.) of games that's already here and MP-only games are excluded.

The 7th Guest series:
The 7th Guest (with uncompressed audio)
Uncle Henry's Playhouse

7.62 series:
7.62 Perezaryadka (7.62 Reload - Russian language only)

Actua Soccer series:
Actua Soccer Club Edition
Actua Soccer 2
Dream Team 98
ranSoccer
Actua Soccer 3

Agarest series:
Agarest Marriage

Agatha Christie games:
Agatha Christie: And Then There Were None
Agatha Christie: Murder on the Orient Express
Agatha Christie: Evil Under the Sun
Agatha Christie: Death on the Nile
Agatha Christie: Peril at End House
Agatha Christie: Dead Man's Folly

A.I.M. series:
A.I.M.: Artificial Intelligence Machine

Airline Tycoon series:
Airline Tycoon 2

(Disney's) Aladdin
Disney's Math Quest with Aladdin
Disney's Aladdin in Nasira's Revenge
Disney's Aladdin Chess Adventures
Legend of Aladdin

Alan Wake series:
Alan Wake Remastered
Alan Wake 2

Alien Breed series:
Alien Breed Obliteration

Alien Nations / Die Völker series:
Alien Nations Gold Edition (including Alien Nations Mission Pack)

Alien Shooter series:
Alien Shooter Revisited
Alien Shooter: Vengeance
Alien Shooter 2: Conscription
Theseus - Return of the Hero
Alien Shooter TD
Alien Shooter - Last Hope
Alien Shooter 2: The Legend
Alien Shooter 2 - New Era

Alien(s) games and Predator games:
Predator 2
Aliens: A Comic Book Adventure
Alien Trilogy
Aliens: Roguelike
Aliens versus Predator 2
Aliens versus Predator 2: Primal Hunt
Aliens vs Predator (2010)
Aliens: Colonial Marines
Aliens: Fireteam Elite
Aliens: Dark Descent

Alone in the Dark series:
Alone in the Dark 1-3 (floppy version with original sound)
Alone in the Dark (2008)
Alone in the Dark: Illumin*tion

American Conquest series:
American Conquest: Divided Nation

Anima series:
Anima Gate of Memories: The Nameless Chronicles

Anno series:
Anno 1602: Im Namen des Königs (In the Name of the King)
Anno 2070
Anno 2205
Anno 1800
Anno History Collection

Aqua Kitty series:
Astro Aqua Kitty

Arcanum series:
Arcanum: The Lost Dungeon of Souls

Ardania series:
Warlock - Master of the Arcane
Warlock 2: The Exiled
Impire
Defenders of Ardania

Arkanoid series:
Arkanoid: Revenge of Doh

ARMA series
ARMA: Tactics
ARMA 2
ARMA 2: Operation Arrowhead
ARMA 3
Arma Reforger

Army Men series:
Army Men: Air Attack
Army Men: Sarge's Heroes
Army Men: Air Tactics
Army Men: Sarge's War

Ash of Gods series:
Ash of Gods: The Way

Ashes series:
Remnant: From the Ashes
Remnant II

Ashes of the Singularity series
Siege of Centauri

Assassin's Creed series:
Assassin's Creed II
Assassin's Creed: Brotherhood
Assassins's Creed: Revelations
Assassin's Creed III
Assassin's Creed: Liberation HD
Assassin's Creed IV: Black Flag
Assassin's Creed Unity
Assassin's Creed Rogue
Assassin's Creed Chronicles: China
Assassin’s Creed Syndicate
Assassin's Creed Chronicles: India
Assassin's Creed Chronicles: Russia
Assassin's Creed Origins
Assassin's Creed Odyssey
Assassin's Creed Valhalla
Assassin's Creed Mirage

Asterix and Obelix series
Asterix and the Magic Carpet
Asterix: Operation Getafix
Asterix: Caesar's Challenge
Asterix & Obelix
Asterix: The Gallic War
Asterix & Obelix Take On Caesar
Asterix: Mega Madness
Asterix at the Olympic Games

Atlantis series:
The Secrets of Atlantis: The Sacred Legacy

Avernum series:
Exile I: Escape from the Pit
Exile II: Crystal Souls
Exile III: Ruined World
Blades of Exile

Bad Dream series:
Bad Dream: Stories

Bard's Tale series:
The Bard's Tale Construction Set

FAR series:
FAR: Changing Tides

Far Cry series:
Far Cry 3
Far Cry 3: Blood Dragon
Far Cry 4
Far Cry Primal
Far Cry 5
Far Cry New Dawn
Far Cry 6

Fatal Fury series:
FATAL FURY FIRST CONTACT

fault series:
fault - StP - LIGHTKRAVTE

Fenimore Fillmore series:
Fenimore Fillmore's Revenge

Fighting Fantasy series:
Fighting Fantasy Classics
Fighting Fantasy Legends
The Warlock of Firetop Mountain
Sorcery!
Sorcery! 2
Sorcery! 3
Sorcery! 4
House of Hell
Caverns of the Snow Witch
Starship Traveller
The Forest of Doom
Appointment With FEAR
Deathtrap Dungeon Trilogy

Five Star General series:
Panzer General
Allied General
Panzer General III: Scorched Earth
People's General: Red Danger Asia
People's General: Red Danger Europe

Flashback series:
Fade to Black
Flashback (2013 remake)

FlatOut series:
FlatOut: Ultimate Carnage
FlatOut 3: Chaos & Destruction
FlatOut 4: Total Insanity

Flight of the Amazon Queen series
Flight of the Amazon Queen: 25th Anniversary Edition

Freedom Planet series:
Freedom Planet 2

Galactic Civilizations series:
Galactic Civilizations IV

Game of Thrones games:
A Game of Thrones: Genesis
Game of Thrones (2012 video game)
Game of Thrones: A Telltale Games Series

Gangsters series:
Gangsters 2: Vendetta

Gary Grigsby's War series:
Gary Grigsby's War in the East 2

Gauntlet series:
Gauntlet
Gauntlet II

GemCraft series:
GemCraft - Chasing Shadows

Genkai Tokki series:
Monster Monpiece

Gex series:
Gex: Enter the Gecko

Ghost Recon series:
Ghost Recon: Desert Siege
Ghost Recon: Island Thunder
Ghost Recon Advanced Warfighter
Ghost Recon Advanced Warfighter 2
Ghost Recon: Future Soldier
Tom Clancy's Ghost Recon Wildlands

Giana Sisters series:
Giana Sisters 2D
Giana Sisters: Dream Runners

Gobliiins series:
Gobliiins 4
GOBLiiiNS5

Goosebumps series:
Goosebumps: Escape from Horrorland
Goosebumps: Attack of the Mutant
Goosebumps: Dead of Night

Gorky series:
Gorky Zero: Beyond Honor
Aurora Watching (a.k.a. Soldier Elite: Zero Hour)

Grand Ages series:
Grand Ages: Rome
Glory of the Roman Empire

Gratuitous Space Battles series:
Gratuitous Space Battles

The Guild series:
The Guild 2: Venice

Gunship series:
Gunship (1986)
Gunship 2000

Hammerwatch series:
Hammerwatch 2
Hammerwatch Anniversary Edition

Hard Reset series:
Hard Reset

Hard Truck series:
Hard Truck
Hard Truck Apocalypse
Hard Truck Apocalypse: Arcade
Hard Truck Apocalypse: Rise Of Clans

Harry Potter series:
Harry Potter and the Philosopher's Stone
Harry Potter and the Chamber of Secrets
Harry Potter: Quidditch World Cup
Harry Potter and the Prisoner of Azkaban
Harry Potter and the Goblet of Fire
Harry Potter and the Order of the Phoenix
Harry Potter and the Half-Blood Prince
Harry Potter and the Deathly Hallows: Part 1
Harry Potter and the Deathly Hallows: Part 2
Hogwarts Legacy

Heart of China series:
Heart of China (EGA/CGA version)

Hearts of Iron series:
Hearts of Iron II: Iron Cross
Hearts of Iron IV

Hello Neighbor series
Hello Neighbor: Hide and Seek

Hegemony series
Hegemony Rome: The Rise of Caesar

Heretic series:
HeXen II Mission Pack: Portal of Praevus
Heretic II

Heroes of Might and Magic series:
Heroes of Might and Magic: A Strategic Quest for Windows 95
Might & Magic Heroes VI
Heroes of Might & Magic III – HD Edition
Might & Magic: Heroes VII

Hero’s Hour series:
Hero’s Hour - Rogue Realms

Hitman series:
Hitman (2016)
Hitman GO: Definitive Edition
Hitman 2 (2018)
Hitman 3 (2021)

Hollywood Monsters series:
Hollywood Monsters

Horizon series:
Horizon Forbidden West

The House in Fata Morgana series
The House in Fata Morgana: Requiem for Innocence

The House of the Dead series:
The House of the Dead (1996)
The House of the Dead 2
The House of the Dead III
The Typing of the Dead
The Typing of The Dead: Overkill

Hugo series:
Nitemare-3D

The Human series:
The Humans: Meet the Ancestors!

Hyperdimension Neptunia series:
Hyperdevotion Noire: Goddess Black Heart
Megatagmension Blanc: Neptune vs Zombies
Superdimension Neptune VS Sega Hard Girls
Cyberdimension Neptunia: 4 Goddesses Online
Neptunia Shooter
Dimension Tripper Neptune: TOP NEP

Hypnospace Outlaw series:
Slayers X: Terminal Aftermath: Vengance of the Slayer

IL-2 Sturmovik series:
IL-2 Sturmovik: Battle of Stalingrad
IL-2 Sturmovik: Cliffs of Dover
Battle over Europe
Combat over Europe
Eastern Thunder
Operation Barbarossa
Ostfront: Decisive Battles in the East
The Last Days: Decisive Battles over Europe

Incoming series:
Incoming: Subversion Expansion Pack

The Incredible Machine series:
The Incredible Toon Machine
Sid & AI's Incredible Toons

Ys series:

Ys Seven (Upcoming)
Ys VIII - Lacrimosa of Dana
Ys - Memories of Celceta (The PC release was limited to China, like the one for Ys Seven before XSEED announced a release for the West. So I do not know, if that should be part of the list.)

Not yet released.
Both games were released exclusively in China. The previous version of the thread was only good for games with an English language option (but did include some non-English expansions).

You are correct. My bad.

Good to know. But Ys Seven is now announced for a GOG release this summer (also on Steam).

I am not sure if they belong to the list:
Heart of China (EGA/CGA version) [1]
Rise of the Dragon (EGA/CGA version) [2]

You can check the screenshots:
[1] http://www.mobygames.com/game/heart-of-china/screenshots
[2] http://www.mobygames.com/game/rise-of-the-dragon/screenshots

They belong here since they can be seen as "superior" versions, other older Sierra/Activision titles (King's Quest and Space Quest) are listed with these versions as well. Thank you!

Added:
Heart of China (EGA/CGA version)
Rise of the Dragon (EGA/CGA version)

If that should be the case, I could also remove the following:
The 7th Guest (with uncompressed audio)
Alone in the Dark 1-3 (floppy version with original sound)
Battle Chess: Enhanced CD ROM
Blood (with uncompressed audio)
Capitalism 1+2 (with CD audio)
Castles II (original floppy version with extra content)
Defender of the Crown: Digitally Remastered Collector's Edition
Fallout 1-2 (uncensored versions)
Heart of China (EGA/CGA version)
King's Quest I: Quest for the Crown (SCI Remake)
King's Quest I: Quest for the Crown (AGD Interactive remake)
King's Quest II: Romancing The Stones (King's Quest II remake)
King's Quest III: To Heir Is Human (AGD Interactive remake)
King's Quest III: To Heir Is Human (Infamous Adventoures remake)
King's Quest IV: Perils of Rosella (AGI version)
King's Quest V: Abscense makes the Heart Go Yonder (floppy version, VGA)
Loom (FM Towns and floppy versions)
Lords of Magic: Special Edition (full audio)
Normality (redubbed US version)
Phantasmagoria 2 (windows version with superior video quality)
Raptor: Call of the Shadows (proper sound quality and fixed controls)
Rise of the Dragon (EGA/CGA version)
SimCity 2000 Special Edition (Windows version)
Space Quest I: Roger Wilco in the Sarien Encounter (VGA Remake)
Space Quest IV: Roger Wilco and the Time Rippers (floppy version)
Space Quest II: Vohaul's Revenge (Infamous Adventures remake)
The Ur-Quan Masters (Star Control 2 remake)
Wing Commander: Privateer (floppy version with Speech Pack)

All those technically belong on a list like these:
https://www.gog.com/forum/general/gog_games_missing_content_broken_games_incomplete_version_variations_etc/page1
https://www.gog.com/forum/general/gog_games_missing_expansions_importantdefinitive_versions_and_informative_bonus_content/page1