When I launch the game, a windows appears saying that there's an update and asks if I was to update it automatically. I click yes and then it launches my ASUS motherboard BIOS updater, instead of the game's... What's going on?

(I can still download the updater in GOG's site or download an update from the developer's site, thankfuly)

you would be smart to ignore that updater with the game and only download gog updates. safer for you and your computer.

The in-game updater doesn't work for me either, but I've never had that message. Sounds like it is the result of two files with identical names and poor pathing/directory logic in the in-game updater.You should report that on Snowbird Games board - they are the developers.

I assume you are using the updater program directly, that is what works.