Some edits, by the way:

Starbound is still missing the OST. However, by using the link I posted above (https://www.gog.com/forum/general/games_that_treat_gog_customers_as_second_class_citizens_v2/post658), you can now download mods from Steam workshop without owning the game on Steam.

Terratech is not just missing its Linux version here. In fact, the game was removed from GOG the same day that it was supposed to come out of Early Access, because the developers wouldn't be adding multiplayer to it, like they did with other platforms.
https://www.gog.com/forum/general/terratech_is_leaving_the_gogcom_storefront
And while it did receive a launch (1.0) patch, it's highly unlikely to have received any of the post-launch updates (I see no such indication on The "what did just update?" thread).

I was looking at SKYHILL, and according to the gogdb and the changelog on the forum, the last version here is 1.1.19. But on steam is 1.1.20 since January 2017.

I don't have the game, so maybe someone who owns it on gog can verify if it's indeed 1.1.19 in their account.

The Lion's Song was included twice in the list as Lion's Song, The and The Lion's Song.

Game says 1.1.19. Not that I care about the patch, but I guess "minor bugfixes and update to new engine" qualifies as useful.

Kyn is missing the latest update which was released on Steam on 20th November, 2018. The patch contains character customisation and some fixes;

New Features:
+ Character Customization.
- Face
- Length
- Pose
- Skin color
- Icon

Fixes:
+ Kiera's Training Achievement Fixed.
Load or start the 2nd level (Shipbuilders by the Sea) and find the loot in order to complete "Kiera's Training" when your return to the village.
+ Minor fixes and optimazations.

The update details can be found here;
https://steamcommunity.com/games/314560/announcements/detail/3398424857463469107

To confuse matters the devs have different patch numbers depending on whether the game is here or on Steam. The latest patch here (#4) is dated 25th August, 2015 and the updated patch on Steam (also called Patch #4) is dated 20th November 2018.

Oh oops. Thanks, fixed now. Removed "Lion's Song, The".

Moved Soundtracks to the new column.

Added:
DROD 4: Gunthro and the Epic Blunder
Kyn
SKYHILL

Updated:
Starbound
TerraTech


(Edited for alphabetical order for no particular reason.)

Updated:
Chroma Squad
Cities in Motion
Clustertruck


Added:
Codename Panzers Series
Cosmic Star Heroine
Crawl
Lumo
Weedcraft

What's the issue with Weedcraft Inc?

Ah right - no 1.01 or 1.02 patch

Oh, that too? Currently it's only on the list for being region-locked in China, which is not the case on Steam.
But it's only just been released here, and we like to give them some time. :)

Fair enough - didn't see any info on region blocks

Nice to read you again Fairfox :)
Any games you'd like to add to the list?