It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

×
...and attempting to launch from Terminal shows the following error:

start.sh, line 20, ./"Fell Seal.x86" location not found

Chances are you're on a vanilla 64-bit LTS or mainstream distro, and the problem is the start.sh was misconfigured during the last patch following the Monsters DLC.

To fix, simply open the game's start.sh with Text Editor (in your GOG Games folder in /Home), go to line 20, and change it to ./"Fell Seal.x86_64".

Done. It will now launch properly from your App Grid just like it used to.