Like they said here:
https://www.gog.com/forum/general/release_shadwen_4b8a0/post76

there is no Map Editor for Windows Gog version. the map editor is only avaiable with Steam version because is linked to Steam Workshop.

If you would like to buy this game, get a DRM-free version and you want the editor I suggest to get the humblebundle version:
https://www.humblebundle.com/store/shadwen

it is also the only place where you can still download the free demo version.
Still if you want to use the editor you will need to use Steam.

If you are on mac or linux there is no editor.

I still think the game is a lots of fun, this post is a warning to avoid lose a big feature like the editor (or to just wait a good discount here on gog).

EDIT. if you own the gog version you can get a steam code by send an email to the support:
http://www.frozenbyte.com/2016/05/shadwen-out-today-on-pc-ps4-os-x-linux/

Doesn't this also mean that user created content from the workshop cannot be played with the GoG version?

this is not clear yet.
I will try that when I will get my steam code.

So if they can't DRM lock it, they don't include it - I wonder why that might be (*cough* paid mods *cough*). In any case, I don't want to support this kind of practice.

After Trine 3 I was really disappointed with the company, but after this, they will never have my money anymore. Sad.