In the current version of TWD commentary are unavailable (cant connect to telltale server). Would GOG consider add them as downloadable addon which make possible to watch offline? Or maybe separate video files?

Absolutely! I was also very disappointed when I saw that the developer commentary is not included in the game but only available on a server (which isn't available anymore).

The store page says: "Behind the Scenes Bonuses: Exclusive looks at the making of the game from the developers, VO artists, and other team members that brought Clementine’s story to life. This includes over 10+ hours of developer commentary and “Return of the Walking Dead” documentary short." Please make these videos downloadable as extras.