Today, i downloaded Slain Back from hell from GOG Galaxy. I launched it through Galaxy, played it for a couple of minutes... But the achievement 'die at least once' never counted for me despite the fact that i did die, and the overlay seemed to be working

I restarted the game, again through Galaxy, and died on purpose just to get the achievement. No effect.

The only way I've found to register achievements on GOG Galaxy 2.0 is to run the launcher as Admin, or else they won't trigger.