Currently it seems according to https://www.gogdb.org/product/1266464371#downloads that the Linux download ist still missing. One thing I found sad in the past was, when buying a game that had an icon for Linux and then you find out the download is missing or even the icon got removed after a short period of time.

So before buying it I would like to know when the download will be ready.

Would like to know too. I always check gogdb to make sure a Linux build exists, too many times did GOG remove the Linux icon right before/after release... Pretty annoying. At least the game has Linux support on Steam so let's hope it will end up here too :)