Apart from various sources that give/leak/enigmatically hint us info on what games are soon to be released, or at least added in the release pipe-line, there's also a vastly under-rated (probably because of its low yield) quasi-official source: the Community Wishlist.

Here are the entries currently marked as "in progress":

Torment: Tides of Numenera - no surprise here
Doom 1 2 and 3 - hmmm, so I'm guessing there are plans on us getting Doom 3 at some point, right? Edit: According to multiple sources, no, it does not mean anything of the sort.
good old warhammer games: Shadow of horned rat, Space Hulk 1&2, Chaos Gate, Dark Omen, Mark of Chaos, etc. - again, no surprise, GoG has gone off acquiring publishing rights for the Warhammer series Edit: According to multiple sources, it only means that some of the games have been released and guarantees nothing about the future of the series on GoG.
Project Zomboid - already available as "In Dev"
Factorio - already available as "In Dev"
Nothing - haha, very funny GoG, *goes sarcastic* but it does sound like a thing you'd be willing to work on - I'd even say it's already seen multiple releases
Standalone Gwent - yeah, very predictable anyway, I'd say
Slime Rancher - already available as "In Dev"
The Long Dark - already available as "In Dev"
No Man's Sky - ups, somebody's been lazy - guess it's the same person that's working on getting "nothing" released
Sins of a Solar Empire: Trinity - again, probably somebody being lazy - don't think we'll see this on GoG since we now have Rebellion

That seems to be it. I know, nothing noteworthy for now, but still it's something to keep an eye on.

I've got to tell you, overall the Community Wishlist is in some serious need of cleanup & consolidation. Sometimes this is done when there's a "business" need for it: see the new #1 wishlist entry for example. But, yes, I know, GoG has bigger issues than this, better things to do.

Still, I wonder if there's anyone specifically in charge of maintaining/managing it from GoG... Fables, you around? Any comment? :)

GOG themselves have dismissed this as nothing more than part of the wishlist being completed. Hence why you won't see any wishlist that mentions only Doom 3 as being in progress. Like the above, GOG has most likely marked this wishlist as in progress, since Chaos Gate and Shadow of the Horned Rat are already here. Similarly, you won't find any individual wishlist for Mark of Chaos, Dark Omen, etc. as being in progress. Finally, GOG hasn't secured the publishing rights for all the Warhammer games, oh no. They have secured the rights for specific Warhammer games, namely those once belonging to SSI/Mindscape. If GOG wants to get their hands on the other Warhammer games, they'll need to convince the respective publishers to do so (EA for Dark Omen, Sega for Dawn of War, Slitherine for Armageddon, etc.)

Now, does that mean that Doom 3 and other Warhammer games won't ever be coming? "Dons enigmatic cowl" Maybe, maybe not. The wishlist being in progress is no indication of that.

All valid points. Ok, granted wishlist entries for multiple games offer plausible deniability and can not be used to divine future releases... *puts on conspiracy theorist hat* in theory.

However, in the case of the Doom and the Warhammer series, I think the current entries have been consolidated as some point, so there's little chance of even finding separate entries for Doom 3, Mark of Chaos, Dark Omen, etc.

Besides, there is still hope in some cases at least.

The merge might have occured, but it hasn't afffected all the series.
Wishlist entries:
Doom 3 (5000+ votes)
Space Hulk (1000+ votes)
Warhammer: Mark of Chaos (1000+ votes)
Warhammer: Dark Omen (3000+ votes)
All present without any hint of "in-progress". The staff member's answer doesn't really say much to me. He hopes that GOG will get Doom 3. Well, I'm pretty sure that Judas once mentioned that he hoped to see Crusader Kings 2 here; that doesn't mean that Paradox will suddenly play ball and release it.

Hmmm, ok, that's certainly interesting and kind of proves your point. It also proves that the Community Wishlist is a disorganized mess badly in need of a "mother in law's" style clean-up. Yes, and I got the same vibe off it. I'm not saying it's an indication of anything really, but consider this - if an entry, even a multiple-item entry, is marked as "in progress" it's safe to assume that at least at some point in time GoG is going to pursue completing it by approaching the said developers/publishers, right? Maybe even nag them to death every now and then if they keep saying no?

All I'm saying is:

Doom 1, 2, 3 wishlist being "in-progress" is no indication that Doom 3 will be coming.
Doom 3 wishlist not being "in-progress" is no indication that Doom 3 won't be coming.
Oblivion wishlist not being "in-progress" is no indication that Oblivion won't be coming.

It's safe to assume that since GOG got Bethesda (or insert your favourite publisher here) on board, they want as many of their games here as possible and they are unlikely to stop at just the initial offerings.

So you're a mind reader of sorts... and it already seems you know way too much for a felis catus.

This in itself is also no indication of any impending release of the #1 issue of a new comic book entitled "Grargar - The Supercat" or its peculiar continued non-existence (as of yet).