I guess we can add "Nova Lands" to the list. Missing DLC, supporter pack (soundtrack and skins) is available on Steam, but not on GOG.

Mount and Blade 2: Bannerlord- newest update from steam (version 1.2.0) since 24/06/2023 and on gog noting for 3 days

Regarding False Skies, it has a Mac build on Itchio, but not on GOG, and the build was disabled on Steam.

Been a few months because the email got pushed back before I could notice it (oopsie), but when I asked the dev about that, he replied: (edit: typo)

Both GOG and Steam were updated to v1.1.5 on June 23.
v1.2.0 on Steam is just a Beta build. It'll probably come to GOG eventually.

Fun fact... I didn't know GOG had Mount and Blade 2 and so when I read this post I copied and pasted "Mount and Blade 2" into the search engine at the gog store front and.... "Sorry, no matches found, browse the whole store?." lol

But when I typed it in.. 1 letter at a time, you could see it come up in a list under the search box so GOG clearly has it.

Not a biggie... but I just wonder why pasting the whole thing in results in "no matches found" when they clearly sell the game. I dunno, if I ran a company and found something like that where it might result in losing just one customer who might've done the same thing I did and gave up I'd want to fix it.

Oh yeah... this is GOG. My purple dot at the top of this very screen hasn't worked right in years. lol

It is Mount & Blade II (roman numerals), not Mount & Blade 2 (arabic numerals).

Well, again, if I ran a company (I don't) and it was possible to lose even a single customer over that little technicality, I'd do some adjusting to my search engine.

But that's just me.

Moto Racer Collection on GOG is missing Moto Racer 15th Anniversary.

Most individuals do not seem to like that particular Moto Racer game. I still think it is alright though worse than the other Moto Racer titles.

I don't know whether this qualifies for this list, but...

Sea Horizon was announced as "coming soon" to GOG in July 2022. The game was released on steam in October 2022. It's still "coming soon" over here.

I have no idea whether it's the developer/publisher that changed its mind, or whether it's GOG --- especially since both the developer and the publisher are Taiwanese.

Body of Evidence was also announced so long ago that everyone thought it would never come out, but then it was shadow-released on Steam in October 2022, while on GOG it will probably remain in limbo for eternity.

Unfortunately we'll have to add "Someday You'll Return" to the list since it is missing its Director's Cut update which is supposed to improve the game a lot and which - according to the devs publisher - won't come to GOG. Of course they don't have a problem with still having the outdated version listed here and getting at least some money from poor GOGers who simply don't know.

https://www.gog.com/forum/general/someday_youll_return_directors_cut_missing_from_gog_because_of_the_publisher

Kingdom's Life

Steam version: v.2.0

https://steamcommunity.com/app/1847600/eventcomments/3818536354782721415

Gog version: v.1.1.2

edit:

Stronghold: Warlords : no Ultimate Edition on Gog

Not sure if this counts:

https://www.gog.com/forum/general/summer_sale_special_endless_space_and_endless_space_2_with_75_discounts_and_another_giveaway_06484/post40

Just want to mention that I made a Firefox Add-On some time ago that fetches data from the spreadsheet and gives a visually loud warning for games that are on the list when browsing GOG. You can find it by searching for "GOG 2nd Class Helper"

I'd say that this is even an advantage :) (compared to having it to buy extra)
But thanks for the headsup. I'll make sure to add the OST to my collection.