GOG Games that do not need GOG Galaxy for currently working online multi-player:

AI War: Fleet Command
Age of Wonders III + Eternal Lords + Golden Realms (DRM: needs serial registration and regular authenticity check)
American Conquest + Fight Back (DRM: needs serial registration)
Baldur's Gate
Baldur's Gate 2
Barony: Cursed Edition
Battle Isle 2
Battle Isle 3
Battle Isle Phase Four: Incubation
Battle Worlds: Kronos (DRM: needs serial registration)
Blood & Gold: Caribbean!
Broforce
Capitalism Plus
Capitalism 2
Celtic Kings: Rage of War
Chaos Reborn (DRM: needs serial registration)
Conflict: Desert Storm
Conquest of the New World
Conquest: Frontier Wars
Daikatana
Darkest Hour: A Hearts of Iron Game
Darkstone
DEFCON (DRM: needs serial registration)
Delta Force
Descent
Descent 2
Descent: Freespace Battle Pack
Diablo Classic (Battle.net)
Die By The Sword + Limb From Limb
Din's Curse + Demon War (DRM: needs serial registration)
Disciples II Gold
Divinity: Original Sin - Enhanced Edition
Divinity: Original Sin 2
DOOM II + Final DOOM
Earth 2150
FlatOut 2 (with gamespy replacement)
Freespace 2
Grim Dawn
Hammerwatch
Icewind Dale
Incredible Adventures of Van Helsing 1
Jagged Alliance 2
Jazz Jackrabbit 2 Collection
Morrowind GOTY (Needs tes3mp mod)
Neverwinter Nights Diamond (DRM: needs serial/keygen & fixes)
Neverwinter Nights 2 Complete
Oh...Sir!! The Insult Simulator
Omerta: City of Gangsters + DLCs
Project: Zomboid
Quake 3 Arena
Screencheat
Serious Sam: The First Encounter
Serious Sam: The Second Encounter
Sins of a Solar Empire: Rebellion
Spellforce 3: Soul Harvest
Star Wars: Jedi Knight II
Star Wars: Jedi Academy
Starbound
STAR WARS: Battlefront II (Classic, 2005) (classic installer)
SWAT 4 Gold (with gamespy replacement)
System Shock 2?
Terraria
Theme Hospital
Titan Quest: Anniversary Edition
Turok
Unepic
Ultimate DOOM, The
Victor Vran
War for the Overworld
WarCraft II: Battle.net Edition
Warhammer 40,000 Gladius - Relics of War
Windward