This is a new one for me, when I start Darksiders Genesis in GOG I get a little error window that says "failed to star the game" (yes that missing t at the end of start is actually missing in that message) but when I click ok on it the game starts....but in the background and it can't be brought to the foreground. I have to minimize the GOG window to see the game and play it.

Now, I would have just ignored the error from then on, since the game works, but I get no achievements. I guess GOG really thinks the game didn't start due to the error and therefore the connection between game and GOG is broken.
Weirdly enough though, cloud saves seem to work despite this.

Oh and when I try to verify the game folder it ALWAYS downloads 134MB of files and takes weirdly long to install them.

Anybody else encountered this and managed to workaround or fix this ? I actually would like the achievements to work so I stopped playing for now.

Ok, I solved it, if you have the same problem, turn off "Access GOG Galaxy features in-game" in the GOG game configuration. That solves it.
Additionally, Achievements were granted retroactively so if you already unlocked some but didn't get them due to the bug, you should get them the next time you play the game.

Thanks for figuring this out. This was also preventing me from playing the game in online co-op. I appreciate the fix.