Might wanna add Strike Suit Zero Director's Cut as I think the gog version is missing this free ship that was made available via update and it's been three years now: http://strikesuitzero.com/blog/the-revenant-the-stealth-suit-available-now/

Also the DLC isn't here either.
http://store.steampowered.com/app/234180/Strike_Suit_Zero__Raptor_DLC/

(TIL Strike Suit Infinity exists too)

The Raptor DLC is included as part of the Director's Cut content. But we don't have the Revenant ship, which I think was the only update this edition ever received. In fact, the company has pretty much gone completely silent for the past 2 years or so.

Another one for your list:
Dying Light

Apparently, to get the new content coming out you need to signup to a third party key seller named Gemly.
Source: https://www.gog.com/forum/general/free_gemlycom_exclusive_dying_light_dlc_steam_only

The way I understood it, Steam users also need to register to Gemly in order to get the new content, so GOG users are no worse than anybody else with this one.

This War of Mine has been updated to 3.0.3 and is now on par with the game's Steam and Humble Store version.
Big thanks to GOG and 11 bit studios for following through with their promise to keep the game up to date.

Sayonara (again), This War of Mine! We hardly knew thee! See you next time when you're 6 months out of date! ;)

In the meantime I got official confirmation that Turmoil was region locked in China at the request of the publisher. I'm considering adding Hyperdimension Neptunia and Dying Light as well, it would be nice if someone could confirm that these are indeed available on Japanese Steam but not on Japanese gog.

What about the DLC? :P

Phew, good thing I didn't have it on the list in the first place. :P

Add Space Rangers HD: A War Apart to the list:

https://www.gog.com/forum/general/the_what_did_just_update_thread/post17161

This is getting ridiculous, I wish these devs, gog or anyone could spell out exactly what the problem is with keeping the gog versions updated.

"Market realities" probably. As in, there are differences between the Steam and DRM-free versions, and maybe there are differences for Galaxy-enabled games for the Galaxy version as well, and actually patching the DRM-free version may be more troublesome (though often they get around that by not providing actual patches and requiring reinstalls, often the norm around here), and Steam makes them more money, so they just focus on that and leave GOG as something that can still provide them with some additional income for no additional maintenance work, calculating that sales lost because of it are largely limited to a handful of people regularly checking the forums, which they determine to ignore unless more of a fuss is somehow made.

It's some time ago now and Arcania misses still the other audio files (especially the original German one). Thiev said a couple of months ago it would be added in future, so did you forget about that or is it a normal time window? Thanks in advance for your reply. :)