Thank you for the correction. I seem to recall other cases in which the answer was something in the lines of "We're not sure how well it will sell" or "Not sure how well it will fit with our core audience", both of which could be paraphrased as "Your product is too niche".
No idea then, but you do have my vote.

It seems GOG uses almost the exact same phrase for a lot of games. This link has a full letter from GOG:
http://segonmedia.com/2014/06/11/good-old-games-dominique-pamplemousse/
And then there's these:
http://www.neogaf.com/forum/showpost.php?p=172667644&postcount=54
http://www.gamasutra.com/blogs/JohnArdussi/20150209/235979/The_Homogenization_of_the_Game_Industry.php
https://forum.thegamecreators.com/thread/214523#msg2560828
http://explorminate.net/2015/10/25/devs-want-more-money-an-exposition/#comment-2163
http://www.rpgcodex.net/forums/index.php?threads/drdungeons-ultizurk-and-madman-thread-madman-released-vote-for-it-on-steam-greenlight.71952/page-24#post-3795602

It doesn't really matter what phrase GOG uses when rejecting a game? Obviously GOG doesn't want to really say what they think and so they could as well just not give any reason at all.

The really important question is how many wrong decisions GOG take, i.e. how many games GOG rejected which sold well afterwards and how many games GOG accepted which sold like crap? Of course their fixed costs per release are very important here, as long as a game makes at least $1 profit after expenses it would probably be worthwhile to do although not be very high priority.

I don't know all this. Probably noone knows the internal business numbers. It might be a bad deal to publish Dungeon Gardians here. (I would probably not buy it although I don't know how many others would like to buy it.)

Is is frustrating that GOG rejects some games? For the devs and the guys who want to buy these games here surely it is. For all others, it isn't really.
I just randomly klicked on a link in your collected and there while reading the line...

"We also released a Kickstarter for the Crystal Quest update and it is going well."

klicked on the Kickstarter and it failed its goals clearly.

Seeing this I think that not only GOG rejected them but also the gamers themselves. However there might be a bit truth in their accusation that GOG rejections are actually not based on sales numbers but on GOG's personal estimation which might just be biased.

So this is the true problem: How to avoid biased rejections??

Maybe it's time to resubmit your game, posting a link to this thread and the wishlist may help. The wishlist has driven up to 139 votes in just 4 days, in my book that isn't so bad.

Link to thread:
https://www.gog.com/forum/general/gogs_frustrating_acceptance_policies/page1

Link to wishlist:
http://www.gog.com/wishlist/games/the_fall_of_the_dungeon_guardians

Voted. Seems like a good fit for GOG. I know I'd buy it. I know GOG is curated, has limited resources, etc. but it would be awfully nice to have a clearer picture of what GOG is turning down and why so the community can speak up if they really want something that's been turned down. Having to find out piecemeal bits of info through the grapevine is frustrating.

Also going to take a moment to reiterate from a similar topic: get those CAVE shmups on here, GOG. Can't believe they turned down Mushihimesama.

Voted... Bitter lol at such GOG "business decisions". Not often do I say but shame on the executive guilty of turning down this, apparently from critics all around the web, great addition from coming here... Stupid.

I'm frustrated that the MacVenture Series (Shadowgate, Deja Vu I & II and Uninvited) were rejected by gog myself. Still baffled by that one.

Right. Thank you for your productive addition to this discussion, or something.

Well, there are quite a lot of people discussing (mostly criticizing) so I take it they all know what they are talking about, or else why the "blah-blah"? I just thought they didn't go changing things because they didn't know GOG was hiring someone with their skills for the position where they could do something.
Now they know and doubtlessly some of them will apply for the position and actually do something to change the situation.

The last time I checked, GoG didn't hire remote, so one would need to live in/near Warsow or be willing to move there.
Has this changed? Sounds like you know everything about GoG's hiring process, so please tell us.

Indeed. One does have to move to Warsaw to work for GOG. Why, is there a problem? After all, in exchange one would get an opportunity to set things straight the way he sees them and get paid for it.

As for GOG's hiring process... I know much, and tell some. :-P

"we think that it appears to be too niche and a bit too small in scale in terms of production value for our core users"

and yadda yadda.....

Even if I'm not the audience for the OP game, I'm quite upset about the "core users" which is a category I think I belong to (here since 2009 with a library of nearly 500 paid games)....

Right now, I'm very angry at all the bullshit this sentence contains as it's insulting for both the applying devs and us, the "core users" which GOG doesn't give fuck about. I'm really tempted to wrire a long e-mail to the GOG staff but I'll probably do it later otherwise it will be filled with insults :o)

Any no blue even caring to respond? :-/

I can't work out if this is a genuine misunderstanding of what I said or an inability to comprehend simple logic. ;)