Every game and date in this thread are tentative.
Game development is an ever changing scape, please don't take anything you read here as absolute certainties. I don't intend for my original posts to have the current hourly up-to-date information, but I will do my best to update them in a timely fashion (daily or at least once a week). I'm always open to rectifications, corrections (especially typos) and suggestions.

Other years: 2017 - 2018 -- 2020

This thread won't see any updates from me past today, January 1st 2020, and will stay as is. The new thread for this year is linked to up above.

Officially confirmed by GOG games (mostly the upcoming games' list from the front page):

1971 Project Helios
Alder's Blood
Asylum
Baldur's Gate 3
Biomutant
Body of Evidence
Brassheart
Builders of Egypt
Close to the Sun (will add it to the calendar in the 2020 thread)
Coffee Talk
Cold Hearts
Creaks
Cris Tales
Cuphead: The Delicious Last Course DLC
Cyber Shadow
Cyberpunk 2077 (I know there's a release date, but I'll put the game in the calendar once we're in the next year, for my sanity)
Descent (2019) (Descent: underground renamed?)
Eitr
ENCODYA
Good Night, Knight
Hollow Knight: Silksong
Iris and the Giant
Iron Harvest
Liberated
LUNA: The Shadow Dust
Ostriv (will probably go IN DEV)
Panzer Dragoon: Remake
Prodeus
Soul Saga
Space Haven
Stellaris: Federations
System Shock Reboot
The Alliance Alive HD Remastered (I'll put it in the 2020 calendar when I'll make that thread)
The Dark Crystal: Age of Resistance Tactics (I'll put it in the 2020 calendar when I'll make that thread)
The Sinking City (I know there's a release date, but I'll put the game in the calendar once we're in the next year, for my sanity)
Vagrus - The Riven Realms
Vampire: The Masquerade - Bloodlines 2
Wasteland 3
Wildfire
X4: Split Vendetta (expansion)
XIII - Remake
Xenonauts 2

IN DEV:

Astrox Imperium
Besiege
Between the Stars
Breathedge
Bright Memory (Episodic game)
Caves of Qud
Chernobylite
Double Kick Heroes
Empires of the Undergrowth
Encased: A Sci-Fi Post-Apocalyptic RPG
Epic Tavern
Exanima
Factorio
Flotsam
Foundation
FoxTail
Freeman: Guerilla Warfare
Godhood
Iratus: Lord of the Dead
Jupiter Hell
Kynseed
Last Leviathan, The
Low Magic Age
Minotaur
Nightmare Reaper
Noita
Postal 4: No Regerts
Project Zomboid
Starpoint Gemini 3
The Guild 3
The Universim
UBOAT
Visage
Wrath: Aeon of Ruin

For me "in dev" is having access to a game to help make it better for release. When it leaves "in dev" is when I'll take it off this list because it released officially.

Unofficially confirmed games: 2019

Since most of the games will come from:
https://www.gog.com/forum/general/games_that_are_unofficially_confirmed_for_release_in_the_future_part_2/
instead of putting the complete link to the source of the information, I will write the number of the post that came from that thread, you'll then need to copy the above link and add what I wrote after "source". There may be two post numbers written after the source, the first number is the original mention of the game and the second number is the latest "informative" post about it. Any information that comes from another source than the above thread will have a direct link provided. This is to be able to fit more titles in one post.

The games are separated by year it was last mentioned with "information" about it. If you feel a game is missing or don't agree with a game being here, tell me why and I might reconsider. People that only give links to information about a game without writing the name and some info won't see it appear here if no one else gives the info.

I would like to thank Andrey82 for giving me permission to keep an unofficially confirmed list of my own and for providing the list in the above thread. If he ever wishes for me to stop providing a list here, I will.

Ara Fell: Enhanced Edition -Source: post10518
Armored Brigade -Source: post10428
Beyond a Steel Sky -Source: post10323
Brave Earth: Prologue -Source: post10005
Broken Lines -Source: post10309
Death Coming -Source: post9943
Dream Engines: Nomad Cities -Source: post9883
Empire of Sin -Source: post9920
King's Bounty II -Source: post10070
Kingdom of Night -Source: post9783
Littlewood -Source: post9943
MechWarrior 5: Mercenaries -Source: post7915 post10518
Mighty Switch Force! Collection -Source: post9945
Mittelborg: The City of the Mages -Source: post9615
Papetura -Source: post10067
Sacred Fire -Source: post6417 post10057
Someday You'll Return -Source: post8372 post9510
Synergia -Source: post10057
Tales of Maj'Eyal: The Lost Land (expansion) -Source: post9644
The Eternal Castle Remastered -Source: post10100 (Thanks WinterSnowfall)
Tropico 6 -Source: post9326
Vaporum: Lockdown -Source: post10292
Victory Project -Source: post9977
West of Dead -Source: post10345

Unofficially confirmed games: 2018

3 minutes to Midnight -Source: post8027 post8506
Agony -Source: post8240 post8351
Army Men: Sarge's War -Source: post7847
Assassin's Creed 2 -Source: post8039
Assassin's Creed 3 -Source: post8039
Atomic Heart -Source: post8871
Backbone -Source: post8974
Black Geyser: Couriers of Darkness -Source: post8321
Bomber Crew -Source: post7843
Civilization V -Source: post8529 post8951
Coffee Crisis -Source: post8266
Din's Legacy -Source: post8729
Farm Folks -Source: post8506
Fluffy Horde -Source: post8742
Full Metal Furies -Source: post7942
Gravity Circuit -Source: post8529
Gray Dawn -Source: post8289 (little chance we'll see it here, click here for some info on why)
Kingdom: Two Crowns -Source: post9020
MegaSphere -Source: post8913
Nighthawks - The Vampire RPG -Source: post8868
Once Upon a Coma -Source: post7979
Realms Beyond -Source: post8503
Red Comrades 2 -Source: post8148
Revenge of the Titans -Source: post8249
Road Redemption -Source: post7803 ( Devs only want to release on Steam at this time at least)
Skyrim -Source: post8039
Stronghold 2 -Source: post8961
Stronghold 3 -Source: post8961
Stronghold Legends -Source: post8961
The Last Night -Source: post7993
The Myst 25th Anniversary Collection -Source: post8156
The New World -Source: post8693
The Unlikely Legend of Rusty Pup -Source: post8871
UnDungeon -Source: post7993

Unofficially confirmed games: 2017

911 Operator -Source: post6250
Army Men: Air Tactics -Source: post7452
Astroneer -Source: post3280 post6166
Black Annex -Source: post1280 post6166
Carnivores Games -Source: post6151
Codename Panzers: Cold War -Source: post3280 post6166
Constructor HD -Source: post1792 and here
Copper Dreams -Source: post6960
Defender's Quest 2 - Source: post159 post6166
Demons Age -Source: post3280 post6166
Dragon Fin Soup -Source: post3280 post6166
Drakensang -Source: post7452 post8863
Entymology -Source: post6356 Is that even a game?
Excubitor -Source: post3280 post6166
Forsaken Castle -Source: post6685
Grav -Source: post3280 post6166 (seems abandoned)
Has Been Heroes -Source: post6506
Hellpoint -Source post6486 post6621
High Hell -Source: post7380
Keen Dreams -Source: post7352
Kill To Collect -Source: post3280 post6166
Life Is Feudal: Your Own -Source: post3280 post6166
Medieval Engineers -Source: post3280 post6166
Northgard -Source: post6265
Paradise Lost: First Contact -Source: post6163
Pathfinder Adventures -Source: post6840
Phoenix Point -Source: post6600
Planetary Annihilation: TITANS -Source: post3280 post6166
Red Comrades Save the Galaxy: Reloaded -Source: post7041
Redeemer -Source: post6227 post6254
Rise & Shine -Source: post7288
Savage Lands -Source: post3280 post6166
Senran Kagura Shinovi Versus -Source: post3280 post6166 (not coming, see why here)
Space Engineers -Source: post3280 post6166
Stardrive -Source: post3280 post6166
Stardrive 2 -Source: post3280 post6166
Stifled -Source: post7452
Stranded Deep -Source: post3895 post6162
Sully: A Very Serious RPG -Source: post3280 post6166
The Franz Kafka Videogame -Source: post6347
The Mandate -Source: post172 post6166 (seems abandoned)
The Neverhood -Source: post699 post6623
The Norwood Suite -Source: post7288
Town Of Light -Source: post3280 post6166
Upwards, Lonely Robot -Source: post3280 post6166
White Day: A Labyrinth Named School - Source: post7341
Wizardry 1-5 -Source: post6695
Worlds Of Magic -Source: post109 post6320 (probably won't come)
Yonder: The Cloud Catcher Chronicles -Source: post7058
Zeno Clash -Source: post3280 post6166

Unofficially confirmed games: 2016 - 2015 - 2014 - 2013

2016

Arakion -Source: post4906
Arcadian Atlas -Source: post2642
BattleTanx -Source: post2904 post4479
Brawl -Source: post4593
Captain Quazar -Source: post2904 post4479
Crush Your Enemies -Source: post4356
Dinocide -Source: post2799
Door Kickers 2 -Source: post2952
Hammer And Sickle -Source: post3811
Her Majesty's SPIFFING -Source: post5495
Insanely Twisted Shadow Planet -Source: post4553
Jay and Silent Bob: Chronic Blunt Punch -Source: post2740
KeeperRL -Source: post2809
Niffelheim -Source: post3280
Nightcry -Source: post2892
Project Resurgence -Source: post4279 (studio dissolved, they *might* still do the game, but I'm not following it anymore)
Reflections -Source: post4279
Rhem 1 -Source: post2638
Rhem 4 -Source: post2638
Scorn -Source: post4618
Shrouded In Sanity -Source: post4279
State Of Decay: Year One Survival Edition -Source: post4219 post5107
Subnautica -Source: post3452
The Bunker - Source: post4880
The Fall Of Lazarus -Source: post4577
The Pit -Source: post3280 (Is this the iOS game or the card game? Unsure if I should count it as released or not)
The Political Machine 2016 -Source: post3280
The Quest -Source: post2465 post2525 (Rejected)

2015

2Dark -Source: post1627
Albert & Otto (Episode 1) -Source: post1985 (Is this game completed? Are they making only poop games now?)
American McGee's Scrapland -Source: post1451
Axiom Verge -Source: post1571
Black Ice -Source: post1222
Call Of Cthulhu: The Wasted Land -Source: post2396 post2399 (Rejected, Unavailable because of licensing problems)
Cogmind -Source: post1771
Decay: The Mare -Source: post1283
Edge Of Eternity -Source: post1321
First Wonder -Source: post2039
Frontiers -Source: post1704
Garbage Day -Source: 2203
Legends of Dawn -Source: post138 post2268 ( GROOT says: Avoid it!) :P
Luckslinger -Source: post1758
Mugen Souls -Source: post2171
Paradise Never -Source: post1373
Psychonauts 2 -Source: post2377
Seven Dragon Saga -Source: post1399 (Cancelled)
Shakedown Hawaii -Source: post2350
Shallow Space: Insurgency -Source: 1612
Son Of Nor -Source: post1479
Stars Beyond Reach -Source: post2088
Tengami -Source: post1313
They Bleed Pixels -Source: post1684
The Consuming Shadow -Source: post1788 post2075 (Rejected)
Vertiginous Golf -Source: post1513

2014

Children Of Liberty -Source: post262
Gex 2 -Source: post1089
Gex 3 -Source: post1089 1101 (Probably not)
Gold Rush! - Source: post764
Legions Of Ashworld -Source: post32 post684 (Rejected?)
Machines At War -Source: post688 post810 (Rejected)
Manos: The Hand Of Fate -Source: post1059
Mims -Source: post1072
Shiness -Source: post541
Star Command Galaxies -Source: post1139
Ultionus A Tale Of Pretty Revenge -Source: post263 post394
Woolfe: The Red Hood Diaries -Source: post828

2013

Cryamore -Source: post138
Deathfire: Ruins of Nethermore -Source: post172 (Cancelled)
Doorways -Source: post108
Fade To Black -Source: post138
Iron Soul -Source: post30
Planetary Annihilation -Source: post138
Routine -Source: post138
Silver Symbiosis -Source: post66 (abandoned project?)
Spud's Quest -Source: post65
Trial By Magic -Source: post138
Two Guys SpaceVenture -Source: post138
Wreckfest (Next Car Game) -Source: post29

Episodic games / Episodic DLCs we're waiting on:

If I'm missing anything point it out.

Hiveswap -- Act: 2 -3 -4
Kentucky Route Zero -- Act: 5

Latest updates


Calendar

Added Ape Out, February 07 2019
Added Shovel Knight: King of Cards (content update) and Showdown (multiplayer), April 09 2019 -Unofficial source and date from the forum-

Officially confirmed by GOG

Added Spellforce 3:Soul Harvest
Added UnderRail Expedition (DLC)

Moved Ape Out to the calendar

Latest updates


Calendar

Added Feudal Alloy, January 17 2019 -Thanks Grargar for the heads up)

Holy shit, doing Gog's work here.

What is the significance of bolded entries?

Come on Dusk

Why are you listing Senran Kagura? XSEED basically cancelled that ages ago by saying they couldn't work around the developer's DRM.