Od dłuższego czasu posiadam na GOG Gothic 2 Gold Edition. Kroniki Myrtany mam również przypisane do konta, niestety w żaden sposób nie mogę ich pobrac/lub uruchomić, Co robię nie tak

Archolos has its own entry in your library (https://www.gog.com/account) and must be installed separately.
Download the offline installers (for the fully localised Polish version, select "polski" from the Language drop-down menu first):

The Chronicles Of Myrtana: Archolos (Part 1 of 2) - 1 MB
The Chronicles Of Myrtana: Archolos (Part 2 of 2) - 1.8GB

Navigate to where the offline installers have been downloaded to, run setup_the_chronicles_of_myrtana_archolos_1.1.3_(52629).exe (or setup_the_chronicles_of_myrtana_archolos_1.1.3_(polish)_(52629).exe if you've downloaded the Polish version) and follow the instructions.

Having Gothic 2 already installed isn't actually required to run Archolos, you just need to have purchased Gothic 2 to be able to add Archolos to your library.