Yeah, I bought the Digital Support Pack, thinking that I would get everything only to later find out that important stuff like the guide and cookbook were in an "Ultimate" edition on steam. They can be bought individually there, but you have to have already bought the base game to buy them. So, it looks like the only way to get them right now and play the game with GoG is to buy the game twice.
Now, as for the Map Pack, it's not clear to me how it's any different from the "Digital World Map" provided in the Digital Supporter Pack, since the reviews on it indicate that it's just a set of pdf files and not interactive at all like the steam page for them claims. However, they do apparently link to https://bildnerverlag.de/medieval-dynasty-map
which is an online map which is interactive. So, we can get at that content without steam, but ideally, we'd have the ability to get the same stuff on GoG that can be gotten on steam.