Idunno. Galaxy of Pen and Paper still doesn't have +1 in the title like on Steam. Which I told them about like a month ago.

Firewatch is missing its soundtrack.

I don't know if this is the right thread but the price of Star Vikings Forever on Steam and Humble store is 7.99€ while on GOG it's 8.99€.

We compare American prices in these cases, as they are the default. We'll confirm it.

Looks like Vampyr is missing DLC: "Vampyr - The Hunters Heirlooms DLC"

It's $9.99 on both GOG and Steam in the US, otherwise it would be for this thread.
(Incidentally, the regular price for the game on GOG in Germany is 9.09€.)

Added
Breach & Clear
Firewatch
SteamWorld Quest: Hand of Gilgamech
Void Bastards
Yono and the Celestial Elephants

Updated Where The Water Tastes Like Wine, missing German text.

Thanks for clarification Snolus.

A note: Realms of Arkania 3: Shadows over Riva had a translation into Spanish (text and voices) in addition to the English and German versions.

Is this version included in a digital version somewhere? Steam doesn't have it. So we're not being treated worse seems like.

Do you mean a disc version? Spanish is not listed on Steam and this (old) discussion thread suggests it's not included there (though I don't speak Spanish). I ask because we usually go by what's available in other online stores.

It is true... for some reason I thought that the Steam version had the Spanish translation, but it has not, sorry.

MegaRace 3 on GOG $6 vs Steam $3?

Not just Steam.