You get an old version (2.18.08) as until today 21. August 2016. The patch 2.18.11 was released on 12. April 2016 if you look at steam ... You can't mod the game, as the mod files are a DLC on steam. No luck in modding, the editor (which is ironicaly shipped with the gog release) depends upon the uncompiled source asset files. So, just don't buy it on GOG! You will waste your money as the developer ("stoic") doesn't care about their GOG customers and GOG doesn't care to provide an good experience to their customers by holding their contract partners responsible for updates and broken features which are solvable.
I bought the Guild series when they where released and so I can tell you a lot about the patching mess and how they never get the game bugfree. The game would be great if you didn't ran from one bug to another and the promised multiplayer part NEVER worked. It was promised on the retail package and you had to learn they will patch it in, and they tried hard but never got it in a working condition. As this game is a really longterm game where you play hours for hours the problems multiply and so you will never end a multiplayer game as it rarely works 2 hours straight. The single player game is also bug ridden but at least it is playable. Save often, keep track of everything to see problems quite fast (you really need to micromanage every aspect especially because of the bugs). I would rather not recommend the title because of the many problems, but as the game has no equal competitors in the genre I would recommend it if you can take quite a bit of frustration and you absolutely love the idea of a simulator of guilds and European cities around 1400 AD. Without bugs it would get 4 to 5 stars from me, but as it is so buggy I could only give it 2 stars.
