Latest updates:

Calendar

Added Batman: The Enemy Within, August 08 2017
Added Rogue Trooper Redux, October 17 2017 -Unofficial date-

Removed Kingdoms and Castles, released July 20 2017

Officially confirmed by GOG list

Moved Rogue Trooper Redux to the calendar

Unofficially confirmed games

Added Mages Of Mystralia -Source: post7127
Re-added The Legend Of Heroes: Trails Of Cold Steel 2 -Source: post6535 post7126

Removed Battle Brothers -Source: post5151 post7058, released July 20 2017

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Thanks for both information.

Latest updates:

Unofficially confirmed games

Removed Mages Of Mystralia -Source: post7127, released July 21 2017

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Week of July 23rd 2017 releases:

July 27 2017
Darkside Detective -Unofficial source and date from developers' Twitter-
Galaxy Of Pen & Paper -Unofficial: and date from [url=https://twitter.com/beholdstudios/status/872094482476539905]developers' Twitter-

July 28 2017
Sundered

Week of July 30th 2017 releases:

August 01 2017
Aztez -Unofficial developer's site- (Make a search for GOG on the page, or go to the end of the comment section. Please remember it might not come here, or might not come here day-1)
Redeemer -Unofficial: and date from [url=http://www.pcgamer.com/redeemer-the-top-down-brawler-about-a-murderer-turned-monk-will-be-out-in-august/]PCgamer-
Slime Rancher -Unofficial: Date from Rock Paper Shotgun-
The Long Dark - Season 1: Wintermute (IN DEV) -Date: Forum / Dev-

August 02 2017
The Legend Of Heroes: Trails Of Cold Steel 1 -Unofficial: Forum-
-Unofficial: Date from [url=https://twitter.com/TacomaGame/status/875033032540868608]developers' twitter-

Edit: Fixed a link.

Latest updates:


Calendar

Added The Coma: Recut, August 29 2017

Unofficially confirmed games

Added Alien Rampage -Source: post7172
Added Phantom Trigger -Source: post7171

Latest updates:


Calendar

Added Sine Mora Ex, August 08 2017 -Unofficial: and date on the [url=https://www.gog.com/forum/general/games_that_are_unofficially_confirmed_for_release_in_the_future_part_2/post7145]forum also-

Removed Galaxy Of Pen & Paper, released July 27 2017
Removed Sundered, released July 28 2017
Removed The Darkside Detective, released July 27 2017

Latest updates:

Calendar

Added , September 26 2017 -Unofficial date from [url=https://www.gog.com/forum/general/games_that_are_unofficially_confirmed_for_release_in_the_future_part_2/post7195]forum-

Moved Redeemer back to the unofficially confirmed list

Removed Slime Rancher (IN DEV), released August 01 2017
"Removed" The Long Dark - Season 1: Wintermute (IN DEV) (I never took the game out from the IN DEV list (it makes it convenient to know what games we're still waiting a release from), but I'm removing it from the calendar portion)

Officially confirmed by GOG list

Moved Hob to the calendar

IN DEV

Removed Slime Rancher, released August 01 2017

Unofficially confirmed games

Added back Redeemer -Source: post6227 post6254

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Monthly update

August 01 2017
Aztez -Unofficial: Game webpage in the comment section-
Redeemer -Unofficial: and date from [url=http://www.pcgamer.com/redeemer-the-top-down-brawler-about-a-murderer-turned-monk-will-be-out-in-august/]PCgamer-
Slime Rancher -Unofficial: Date from Rock Paper Shotgun-
The Long Dark - Season 1: Wintermute (IN DEV) -Date: Forum / Dev-

August 02 2017
The Legend Of Heroes: Trails Of Cold Steel 1 -Unofficial: publisher's Twitter-
Tacoma -Unofficial: Date from developers' twitter-

August 08 2017
Batman: Enemy Within
Hellblade: Senua's Sacrifice
Sine Mora EX -Unofficial: and date on the [url=https://www.gog.com/forum/general/games_that_are_unofficially_confirmed_for_release_in_the_future_part_2/post7145]forum also-

August 09 2017
Fictorum -Unofficial: Forum-

August 10 2017
Phantom Trigger -Unofficial: and date from [url=https://twitter.com/tinyBuild/status/892469439261589504]publisher's twitter-

August 11 2017
Sudden Strike 4

August 15 2017
Ken Follett's The Pillars Of The Earth

August 17 2017
Darkwood -Unofficial: and date from the [url=https://twitter.com/TheAcidWizard/status/889143593029566466]developer's twitter-

August 22 2017
Marvel's Guardians of the Galaxy - Episode 3

August 25 2017
The Journey Down: Chapter Three -Unofficial: and date from [url=http://www.metacritic.com/browse/games/release-date/coming-soon/pc/date]Metacritic-

August 29 2017
Absolver
Hello Neighbor
The Coma: Recut

Edit: Link correction

Latest updates:

Calendar

Removed Tacoma, released August 02 2017
Removed The Legend Of Heroes: Trails Of Cold Steel, released August 02 2017

IN DEV

Removed The Long Dark - Season 1: Wintermute, released August 02 2017

Latest updates:


Unofficially confirmed games

Added Milanoir -Source: post7206
Added Treasure Adventure World -Source: post7214

Removed Alien Rampage -Source: post7172, released August 03 2017

Latest updates:


Calendar

Added Ys Seven, August 30 2017 -Unofficial: and date from the [url=https://www.gog.com/forum/general/games_that_are_unofficially_confirmed_for_release_in_the_future_part_2/post7237]forum-

Moved Batman: The Enemy Within to the officially confirmed list

Removed Fictorum, released August 09 2017
Removed Hellblade: Senua's Sacrifice, released August 08 2017
Removed Sine Mora Ex, released August 08 2017

Officially confirmed by GOG list

Added Batman: The Enemy Within

Unofficially confirmed games

Added Orwell: Ignorance Is Strenght -Source: post7230

Moved Ys Seven -Source: post7108 & publisher's Twitter to the calendar

Latest updates:


Officially confirmed by GOG list

Removed Batman: The Enemy Within,released August 10 2017

Unofficially confirmed games

Removed Phantom Trigger -Source: post7171, released August 10 2017

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I will stop checking on Project Resurgence as the studio behind it was dissolved, the game *might* still be made, if anyone keeps an eye on it and sees its actively being worked on talk about it either here or in the unofficially confirmed thread and I'll bring it back on my list. For now I'll tag it as not being worked on anymore in my unofficial list 2016. More information.

Latest updates:


Calendar

Added >observer_, August 15 2017

Removed Sudden Strike 4, released August 11 2017

No links since GOG has problems with me editing or posting my latest updates otherwise. So light version now, will try to edit the OP tomorrow.

Latest updates:


Calendar

Will add when possible Songbringer, September 01 2017 -Unofficial: forum (post3506) and date from the developer's twitter-

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Minecraft Story Mode Season 2 Episode 2 is coming out tomorrow (August 15 2017). Info taken from Telltale's twitter. For those interested.

It is still sketchy as to which posts I can edit and which I can't. So the OP and other relevant posts will be edited when it is possible for me to do so.

Latest updates:


Calendar

Removed Ken Follett's The Pillars Of The Earth, released August 15 2017
Removed >observer_, released August 15 2017

Unofficially confirmed games

Will add when possible Gauntlet: Slayer Edition -Source: post7243

Latest updates:


Calendar

Added Raiden V: Director's Cut, October 05 2017 -Unofficial: forum-

Officially confirmed by GOG list

Added Into The Breach

Unofficially confirmed games

Will add when possible Gauntlet: Slayer Edition -Source: post7243
Will add when possible Shadows: Awakening -Source: post7270

Removed Darkwood -Source: post1144, released August 17 2017

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I've seen the post by murkki in the unofficially confirmed thread (post7268) saying Mandate is probably cancelled, but I'll wait til the end of the year to write that it is in my list. I'm doing this since there are some games I was looking on Twitter that seemed to be "dead" and just recently had information given. Some games that seem "dead" (if anyone can confirm they are and give a source; or confirm they're still being worked on (with a source also):

Defender's Quest 2 -- Twitter followed: larsiusprime
Grav -- Twitter followed: playgrav
Paradise Lost: First Contact -- Twitter followed: ashtreeworks
Routine -- Twitter followed: LunarSoftware

I backed that one on Kickstarter, and AFAIK it's not dead at all. It's simply that the devs don't post many updates about it. They released a Windows+Mac demo back on January, and since then they posted a content and gameplay-related update back in May. A new update is probably on its way soon.


PS: I know it's been on development since almost forever, but I doubt (and hope not) that Defender's Quest 2 is dead either.