The list had been lost since GOG deleted all GOGmixes without warning their creators, but thanks to the existence of a single snapshot on Wayback Machine, I've been able to recover it:

ABSOLVER
- Labyrinth Prospect Mask and Uring Priest Set DLC (preorder exclusive for PC and PS4)
- Red Essence Mask and Shabu Commander Set DLC (PS4 exclusive)

AEGIS DEFENDERS
- Kickstarter exclusives (Kickstarter-only turret for Bart, food-themed skins for Bart's defense block and Clu's explosive trap, Aegis Arcana artbook and world guide, alternate skin for Bart, alternate skin for Clu, alternate Kobo companion sprite)

AGE OF WONDERS 3
- Elven Resurgence Scenario DLC

BANNER SAGA
- Insane Viking Pack DLC (Tryggvi character, Tryggvi's Necklace item)

BANNER SAGA 3
- Dredge Ally Pack DLC (Kivi character, Sculptor’s Tools item, Shield Cleaver heroic title, Dredge Heraldry banner, exclusive overture track by Austin Wintory)

BLACKWELL EPIPHANY, THE
- Soundtrack, Videos (interviews w/voice actors Abe Goldfarb & Rebecca Whittaker, tour of NYC w/Dave Gilbert) (available after release only via Humble widget on WadjetEyeGames.com, bundled with that version of the game, no longer available on GOG)

COSSACKS 3
- Early Bird DLC (mini-campaign, two unique units, special multiplayer badge) (gained by playing the game online during the first two weeks after release)

CROOKZ - THE BIG HEIST
- Alternate Outfits DLC
- Soundtrack

DILUVION
- The Deringer Submarine DLC
- Captain's Journal (Lore & Mythology)
- Kickstarter Exclusive Submarines (Spiny Death, Clarity's Fall, Noble Folly, Redeye's Revenge)

DUNGEONS 3
- Throne Room DLC
- Soundtrack

GRAND AGES: MEDIEVAL
- Ancient Wonders DLC
- Camelot DLC
- Soundtrack

HELLBLADE: SENUA'S SACRIFICE
- Comic

HELLMUT: THE BADASS FROM HELL
- GOG Galaxificator DLC

KYN
- Legendary Fire Master Set DLC

LITTLE NIGHTMARES
- Fox Mask DLC, Tengu Mask DLC (both available for free to registered users on Bandai Namco store for a limited time, no longer available)

NO MAN'S SKY
- Horizon Omega Ship DLC

PILLARS OF ETERNITY II: DEADFIRE
- Fig exclusives (Mātakau, Obsidian Wurm Pet, Cloak of the Obsidian Order)
- The Captain's Footlocker DLC (St. Drogga's Skull; Beakhead, the White Hawk Pet; The Black Flag)

RAILWAY EMPIRE
- Super Hudson Locomotive Skin DLC
- Soundtrack

ROGUE WIZARDS
- Kickstarter exclusives (weapons, armor, trinket, dragon whelp pet, advanced wizard tower, and owl pet)

SEVEN: THE DAYS LONG GONE
- Shadowhand Armor DLC

SHADOW WARRIOR 2
- Razorback Chainsaw Katana DLC

SUDDEN STRIKE 4
- Battle of Kursk Skirmish Map DLC
- Artbook
- Soundtrack

SURVIVING MARS
- Soundtrack

TORMENT: TIDES OF NUMENERA
- Backer Outfit Set DLC (Mindforged Synthsteel Plating, Witcrafted Shapestone Harness, Willwoven Adamant Regalia) (not available on GOG, but the file from the backer site is compatible with the GOG version of the game)

TYRANNY
- Fatebinder's Seal and Commander's Will DLC

URBAN EMPIRE
- Half Timbered Building Pack DLC
- Two Sides of the Same Coin DLC
- Soundtrack

VICTOR VRAN
- Wanderer Outfit (Early Access exclusive, not available on GOG)
- Samurai Outfit (PS4 exclusive)
- Trooper Outfit (Xbox One exclusive)

VIKINGS - WOLVES OF MIDGARD
- Extra tattoo, hairstyle, clan symbol, beard style for male characters, and tribal jewelry for female characters
- Artbook
- Soundtrack

YS VIII: LACRIMOSA OF DANA
- Adol's Adventure Essentials DLC (Adol's Silver Armor, Adol's Adventurer Clothes, Adol's Survival Kit)
- Mini Art Book
- Soundtrack Sampler