Careful about crossing over to console territory. We might be adding stuff like Saints Row 2, Tomb Raider Underworld and other games with questionable PC ports or console-exclusive content.

Pricing shouldn't be a legitimate issue unless there is an unreasonable base price discrepancy.
So if a distributor took a larger percentage of the sale price, you would expect that cost to be passed on; but if it was lower then having any price increase would be unreasonable.
eg. say steams cut is 40% on a $10 game, but GOG's cut was 20%; then you shouldn't be miffed necessarily at a level price of $10 even if realistically it should be sold here at $8; but if it was sold at GOG for $10.10 it would be completely unreasonable because they would of already got more than their fair share as negotiated with the developer in order to host it and sell it.

Then maybe it'd be interesting to add NIS America's games? They support regional prices on Steam, but here, it's near non-existent. And according to an user in the publisher's Discord group, they always say something like "we will check", and nothing ever happens.

Some resolved issues:

Industria: GOG version has been updated to 1.0.7, so it should be on par with the Steam version again
Wasteland 3: The missing "Colorado Survival Gear" DLC is now available as a standalone DLC on GOG.

UNOFFICIAL ISSUES ROUNDUP 5 January 2022

Here are all issues posted since the last official list update on 29 November 2021:

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NEW ISSUES
Ashes of the Singularity: Escalation (2928): Missing "Dawn of the Singularity" eBook DLC
Alien Shooter 2 (2869): Missing Russian language update
Banner Saga 3 (2861): "Deluxe/Legendary Items" DLC are only available as part of the Deluxe/Legendary Editions on GOG, while Steam sells them as separate DLCs as well
Beyond Good & Evil (2900): Missing controller support in GOG version
Blasphemous (2993): GOG OST is missing new tracks from the free DLCs, which have been added to the Steam OST
Demon Turf (2950): Not outdated right now (?), but the devs have stated that the upcoming (?) 1.1 patch will include a level editor which is going to be Steam-exclusive
Demon Turf: Trials (2950): Game not available on GOG
Dungeon Town (2933): Missing Japanese language
Dusk (2931): Missing "The Goodies" DLC
Epistory - Typing Chronicles (2875, 2888): Missing updates since November 2018 - The devs have been informed and are planning to update the game some time during winter 2021/2022
Flynn: Son of Crimson (2923): Missing 1.1 update
Ideology in Friction (2933): Missing Japanese language
Iris and the Giant (2942): Missing updates on GOG - GOG version is 1.1.1, Steam version is 1.1.4 (released 18 May 2020)
ITTA (2978): Missing update since 18 May 2020
Kingsway (3005): Missing major November 2021 update - dev has been informed
Mechajammer (2907): Missing OST
Nanotale - Typing Chronicles (2875): Missing OST
Neon Abyss (2937): Missing updates since 20 September 2021
Police Stories (2941): Missing update 1.4.3 (released 28 October on Steam), which includes lots of new content (campaign, better AI etc.)
Shadow Tactics: Blades of the Shogun: Aiko's Choice (2917): Missing Linux build
Steel Division 2 (2984): Generate Mod tool doesn't work in the GOG version because it requires the "steam_api64.dll" file included in the Steam version
Spellforce 3 (2919): Spellforce 3 has received the "Reforced" update on GOG. While it seems the game still doesn't recognize if you've got the DLCs installed (which previously meant you can't use all factions in multiplayer/skirmish), there have been reports that Dark Elves and Dwarves (from DLC 1) now work in multiplayer/skirmish, but not Trolls (from DLC 2)
Sphinx and the Cursed Mummy (2922, 2982): Three missing updates
Succubus (2995): Missing some (free?) DLC and updates, including Christmas content.
Summer Memories (2933): Missing Japanese language
The Binding of Isaac: Rebirth (2976): Missing Mac and Linux builds
The Binding of Isaac (2990): Missing OST on GOG
The Falconeer (2998, 3006): Missing free "Game Manual" DLC on GOG - the dev has contacted the publisher to rectify this
The King of Fighters '98 Ultimate Match Final Edition (2983): Missing Rollback major update
Treasure Hunter Claire (2933): Missing Japanese language
Turnip Boy Commits Tax Evasion (2880, 2882): Missing Linux build, devs claim this is because the "GOG SDK breaks completely when building for Linux" and they would rather not have a Linux build on GOG instead of providing one that doesn't have all features, such as achievements
VenusBlood Frontier (2933, 2992): Missing Chinese language support - Chinese is Steam-exclusive according to the dev's Discord
VenusBlood Hollow (2933, 2992): Missing Chinese language support - Chinese is Steam-exclusive according to the dev's Discord
Wartile (2863): Missing hotfix 1.2.114.0
Wytchwood (2927): Missing OST
Yestermorrow (2907): Missing OST
Zombie Shooter 2 (2866, 2871, 2873): Missing updates and features - The dev has confirmed that they have abandoned all of their games on GOG

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NEW ISSUES REGARDING GAMES REMOVED FROM GOG
Human Fall Flat (2960): Missing levels that were only added on Steam

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EXISTING ISSUES VERIFIED
King's Bounty GOTY (2490, 2933): Confirmation that the russian language version for the Armored Princess campaign isn't included in the GOG version

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RESOLVED ISSUES
Death's Gambit (2971): has received the "Afterlife" major update
Industria (2862, 3023): has received update 1.0.7
Othercide (2946): has received the latest update
Wasteland 3 (3023): "Colorado Survival Gear" DLC is now available as a separate DLC on GOG

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MENTIONS WHICH MAY NOT BE FIT FOR THE LIST AND REQUIRE ARBITRATION
Anno 1404 (3013): Is available on Steam as "Anno 1404 - History Edition", which is technically a different product from the core game. See 3018 for context (there are other missing editions of games on the list which are technically separate products). Should this be included on the list?
Anno 1701 (3013): See Anno 1404, but for uPlay
Kao the Kangaroo Trilogy (2939, 2940): The Zoom Platform version has a manual custom-made for that version. Should this be included on the list?
Mary Skelter 2 (3012): PC version is censored on all stores, compared to the console version. Should this be included on the list?
NIS America's catalogue (3022): The company's games don't (?) support regional pricing, in contrast to Steam. Should this be included on the list?
Saints Row III Remaster (2973): Offline players can't save graphics options and "some other stuff", but 2974 seems to imply this is also the case for the Steam version. Should this be included on the list?
Samorost 2 (3020, 3021): It's been mentioned that the only issue for this game on the list is price-related. Should it be taken off the list?
The Binding of Isaac (3001, 3002): Posts claiming some (?) mods don't work with the GOG version. Should this be included on the list and can it be verified?
Various games (2953): Some examples of games where the Linux and Mac versions are outdated compared to the Windows versions, so technically, the games are missing patches. Do we track this here?

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Please note that this list does not include posted issues if a later post says the issue got resolved. I also didn't check whether any of the issues mentioned have since been resolved unless there's another post explicitly saying they got resolved.

Thanks everybody for contributing! If I missed anything or you have more up-to-date info on these issues, feel free to add your info to the thread.

EDIT:
Deleted the entry for Chasm as per soosgjr's post 3025 - thanks soosgjr!

Chasm has been updated to version 1.081 on the 23rd on all three platforms, so that should be up to date with Steam now.

do most of the problems stem from GOG having a tiny marketshare thus devs don't want to bother with us little people? Interesting that Duskdev was willing to provide an exclusive multiplayer level but can't be bothered to give us "The Goodies" DLC

But they did. It's mentioned on the store page and on GOG Database, it's just part of the game (like free extras on GOG usually are) rather than being listed as a separate free DLC.

EDIT: Or maybe they didn't? Apparently the Goodies download on GOG has been there forever, so maybe that's a totally different thing.

The goodies available on GOG are different from the "The Goodies" DLC released on Steam.

GOG's "Wallpapers", "Icons" and "Goodies" zip files together are about 189 MB in size (according to Windows), while the DLC on Steam is about 2.1 GB according to my Steam downloads (I have slow internet at the moment, so I won't fully download it to compare the file size directly).
The three files on GOG contain wallpapers, icons, two txt files and 3 early builds ("pre pre pre pre pre pre etc alpha", "Pit" and "E3 2016 Demo). Compare this to the files list shown on the Steam store page:
https://cdn.akamai.steamstatic.com/steam/apps/1846130/extras/where_da_goodies_jpg_steem.jpg?t=1639366512

As far as I can tell, Dusk has so far received updates very reliably on GOG, but given that Szymanski and New Blood haven't released the "Dusk '82" demake on GOG and they haven't added the free files from the Steam "Goodies" DLC to the already existing goodies on GOG, I think this is a worrying development.

The Steam goodies are 3.66 GB.

Hello Exsurgent, add to the list The King of Fighters 2002 UM. The GOG version does not have rollback netcode.

The question is whether they're outdated compared to the same versions elsewhere. If yes, then of course, why would it make any difference? If no, then it's the Mac/Linux users that are treated as 2nd class everywhere, not GOG users; definitely a problem, but not for this thread I guess.

Thanks for checking, but that issue is already on the official list (line 480 in the GDocs document), so I didn't add it to the unofficial roundup.

I'll let ElTerprise determine whether these games go on the list - though it would probably be helpful to them if someone can check if the Linux/Mac builds of these games are out-of-date on Steam as well. I'll be honest: I have no idea how to check for Linux/Mac build numbers on SteamDB and the person who mentioned this issue in 2953 didn't say anything about the Steam builds.

Just got this back from Playism concerning the missing update for La-Mulana 2. Just hoping that they follow through with this.

Hello there,

Thank you for contacting us.
We are planning to update the GOG version, but it will take some time.
We apologize for the inconvenience and ask for your patience.

Best regards,
PLAYISM support team