Edit: The GOGmix is now dead. The link below is an archived version (16th June 2018). The forum thread will require a bit of modification plus potential linking to an external source, to explain what is the use for each serial.

I was browsing the GOGwiki's game pages and I happened to take a look at Two Worlds. That's when I noticed the blurb at the top of the page that it required a serial number for its multiplayer, something that is not disclosed in the game's description, neither does its serial number appear under the installer, as is the case with the newer games. Searching with the word "MultiplayerKey" in the GOGwiki took me to this page, which is informative, dated and somewhat wrong (Neverwinter Nights 2 serials are automatically generated and you don't need to call support for them).

Further investigation brought me to kaliy's GOGmix, which, while equally informative, was also dated (Sacred Gold no longer requires a serial now that its official servers are down). Since GOG doesn't list all the games that come with serial numbers, I decided to create a thread, along with a GOGmix, that lists all the games that come with serial numbers and all games that need serial numbers for multiplayer, but are not automatically generated by GOG (you need to contact support to send you the serial).

Boring prologue aside, the list!

Age of Wonders 3 (IAmSinistar, Wurzelkraft)
Anno 1404: Gold Edition
ArmA Gold Edition*± (JogsterXL)
ArmA: Cold War Assault*± (JogsterXL)
ArmA 2: Combined Operations± (mqstout)
American Conquest + Fight Back* (IAmSinistar, Tarnicus)
Battle Worlds: Kronos
Blood & Gold: Caribbean! (kbnrylaec)
Chaos Reborn (mrkgnao)
Cities in Motion
Cossacks 3
Crashlands
Darkest Hour: A Hearts of Iron Game (JudasIscariot)
Defcon (mqstout)
Desktop Dungeons (IAmSinistar)
Din's Curse (mqstout)
Don't Starve
Don't Starve: Reign of Giants (Wurzelkraft)
Don't Starve: Shipwrecked (JudasIscariot)
Drox Operative (Maxvorstadt)
Eador: Masters of the Broken World
Earth 2160*
Empire Earth 2 Gold Edition* (IAmSinistar)
Empire Earth 3* (IAmSinistar)
Europa Universalis: Rome Gold (mrkgnao)
Factorio (kbnrylaec)
Far Cry 2 (Tarnicus)
For the Glory: A Europa Universalis Game (JudasIscariot)
Frozen Cortex
Frozen Synapse
Frozen Synapse: Red
Galactic Civilizations I: Ultimate Edition (Tannath)
Ground Control 2: Operation Exodus*
Hearts of Iron III (ElTerprise, Wurzelkraft)
Heritage of Kings: The Settlers (mqstout)
Heroes of Annihilated Empires (mqstout)
Heroes of Might and Magic® 5: Bundle
I.G.I. 2: Covert Strike* (JogsterXL)
King Arthur's Gold (Wurzelkraft)
Majesty 2 Collection (mrkgnao)
Mount & Blade: Warband (MadyNora)
Mount & Blade: Warband - Napoleonic Wars (MadyNora)
Mount & Blade: Warband - Viking Conquest
Mount & Blade: With Fire and Sword (MadyNora)
Multiwinia (getlogan)
Neverwinter Nights Diamond
Neverwinter Nights 2 Complete
Painkiller Black Edition*
Pillars of Eternity
Prison Architect (hummer010, Wurzelkraft)
Prison Architect: Name in Game
Quake III: Gold (SCPM)
Race the Sun (mqstout)
Rise of the Triad (2013)
S.T.A.L.K.E.R.: Call of Pripyat
S.T.A.L.K.E.R.: Clear Sky
S.T.A.L.K.E.R.: Shadow of Chernobyl
Sacrifice*
Sengoku (JudasIscariot)
Settlers 2: 10th Anniversary (mqstout)
Settlers 3: Ultimate Collection (mqstout)
Singularity (kbnrylaec)
SpellForce Platinum* (IAmSinistar)
SpellForce 2: Shadow Wars*± (IAmSinistar)
Spore Collection
Surviving Mars (olthor)
Tales of Maj'Eyal (Tarnicus)
Tales of Maj'Eyal: Ashes of Urh'Rok (IAmSinistar)
Tales of Maj'Eyal: Embers of Rage (JudasIscariot)
Tales of Maj'Eyal: Forbidden Cults (kbnrylaec)
Tropico 3 Gold Edition
Two Worlds Epic Edition (F4LL0UT)
Two Worlds 2: Epic Edition
Tyranny
Unreal Tournament 2004 Editor's Choice Edition* (InkPanther)
Victor Vran
Wargame: European Escalation (Tannath)
X2: The Threat (JudasIscariot)
X3: Reunion (JudasIscariot)
X3: Terran War Pack (JudasIscariot)
X Rebirth (JudasIscariot)
Zombasite

Obsolete
Cossacks Anthology (InkPanther, mrkgnao)
Empire Earth Gold Edition (Tarnicus)
Far Cry (Sachys, Tarnicus)
Sacred Gold (Sachys, triock)
The Witcher 2: Assassins of Kings (Gydion, Wurzelkraft)
World in Conflict: Complete Edition (kbnrylaec)

*The game doesn't come with a serial by default and you'll need to contact support to get one.
±The game has been removed from sale, but you can still request a serial from support. (Thanks for the info IAmSinistar!)

You can find the GOGmix below:
https://web.archive.org/web/20180616214401/https://www.gog.com/mix/gog_games_with_serial_numbers?ccr=y

Feel free to point out any inaccuracies/mistakes/omissions. Cheers!

The following don't come with serial numbers. Don't know if you need one for MP, never tried it.
- Far Cry
- Two Worlds Epic Edition
- Unreal Tournament 2004 Editor's Choice Edition

Sacred Gold wants CD keys for online multiplayer, but not LAN.

Far Cry only wants a server password.

Edit: and as above no serial numbers in library.

I guess it's no longer needed as the official servers are down, right?

I can also confirm that the following come with serial numbers:
- Galactic Civilizations I: Ultimate Edition
- Wargame: European Escalation

All the three aforementioned games were released on GOG before 2012. Prior to that, games with serial numbers didn't have them automatically generated and you had to contact support to send you the key. I can't personally confirm it, though, which is why I ask for confirmation. Thank you. Added to the list.

You do. I think when you enter the Network Multiplayer menu you will be prompted to enter a serial by default (in the GOG version that is, IIRC the original retail version required a serial + online verification even for singleplayer, much like in case of Earth 2160). I don't remember when I entered mine but I requested a serial for Two Worlds last year, I wouldn't have done that if I hadn't encountered such a prompt upon trying to play multiplayer.

Edit: I thought I...

I checked the following and they don't come with serial killers:

- American Conquest + Fight Back
- Empire Earth Gold Edition
- SpellForce Platinum±
- SpellForce 2: Dragon Storm±
- SpellForce 2: Shadow Wars±

And will change that as I am testing them now.

- American Conquest + Fight Back requires a code for internet multiplayer. The form requested my full name, email address, ICQ number(I'd have to go back to the 90s to find it! :P) and a bunch of other crap.

Is that a good thing or a bad thing? :P Apparently, you need to contact support to get the serial number. Since I don't have the game, though, I can't personally confirm it.

Crikey! So I need to actually install these games and attempt to play them? You're getting me out of my comfort zone Gragar. Those games have been collecting a thick coating of dust on my shelf for some time now.

If you don't hear back from me, the serial killers have won!

Edit: to clarify, I have a bunch of the games you listed and will start installing and testing them now. I could do with a distraction from my head at 2am :)

Think of it as an opportunity to clean the dust.

You can't let them win!

I edited my post above as:

- American Conquest + Fight Back requires a code for internet multiplayer. The form requested my full name, email address, ICQ number(I'd have to go back to the 90s to find it! :P) and a bunch of other crap.

Onto the next one...

An obvious question, but can you personally confirm this?

I got both keys by contacting support. For Cossacks it was just simple plain text in an email well over a year ago (there's even multiplayer notice on the game page) and for UT2004 it was in form of a .reg file. You can also look at specific subforums - there are some topics about keys for multi.